[DOCS] Clarified $adjrequrl$ error page variable

This commit adds a note to the documentation that the `$adjrequrl$` error page variable will only function if the error occurred during a call to `fs.readFile`.

docs/features/wire.rst

@@ -71,24 +71,24 @@ In error pages with MIME type ``text``, information about the server and error m
 
 The possible variables that may be used within an error page are described below. Each variable may be used zero, one, or multiple times.
 
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| Variable         | Description                                                                                                                                                                                      |
-+==================+==================================================================================================================================================================================================+
-| ``$requrl$``     | The request URL                                                                                                                                                                                  |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$adjrequrl$``  | The request URL with \"index.html\" appended, if ``wireConfig.indexRedirect`` is set to ``true``. Otherwise, this is the same as ``$requrl$``.                                                   |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$osplatform$`` | The operating system platform of the server. See `os.platform() <https://nodejs.org/api/os.html#os_os_platform>`_.                                                                               |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$ostype$``     | The operating system type of the server. See `os.type() <https://nodejs.org/api/os.html#os_os_type>`_.                                                                                           |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$osversion$``  | The operating system release of the server. See `os.release() <https://nodejs.org/api/os.html#os_os_version>`_.                                                                                  |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$port$``       | The port on which the Wire server is listening.                                                                                                                                                  |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$errcode$``    | The code of the error. See `error.code <https://nodejs.org/api/errors.html#errors_error_code>`_.                                                                                                 |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$errno$``      | The number of the error. See `error.errno <https://nodejs.org/api/errors.html#errors_error_errno>`_.                                                                                             |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
-| ``$errmessage$`` | The error message. Note that this may contain information such as the absolute path to a resource on a server. See `error.message <https://nodejs.org/api/errors.html#errors_error_message_1>`_. |
-+------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| Variable         | Description                                                                                                                                                                                                             |
++==================+=========================================================================================================================================================================================================================+
+| ``$requrl$``     | The request URL                                                                                                                                                                                                         |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$adjrequrl$``  | The request URL with "index.html" appended, if ``wireConfig.indexRedirect`` is set to ``true``. Otherwise, this is the same as ``$requrl$``. This variable will only function in the event of an ``fs.readFile`` error. |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$osplatform$`` | The operating system platform of the server. See `os.platform() <https://nodejs.org/api/os.html#os_os_platform>`_.                                                                                                      |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$ostype$``     | The operating system type of the server. See `os.type() <https://nodejs.org/api/os.html#os_os_type>`_.                                                                                                                  |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$osversion$``  | The operating system release of the server. See `os.release() <https://nodejs.org/api/os.html#os_os_version>`_.                                                                                                         |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$port$``       | The port on which the Wire server is listening.                                                                                                                                                                         |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$errcode$``    | The code of the error. See `error.code <https://nodejs.org/api/errors.html#errors_error_code>`_.                                                                                                                        |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$errno$``      | The number of the error. See `error.errno <https://nodejs.org/api/errors.html#errors_error_errno>`_.                                                                                                                    |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| ``$errmessage$`` | The error message. Note that this may contain information such as the absolute path to a resource on a server. See `error.message <https://nodejs.org/api/errors.html#errors_error_message_1>`_.                        |
++------------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+