[doc] Document the builtin configuration

docs/config_reference.rst

@@ -2425,3 +2425,18 @@ These are two examples of YAML logging configuration that uses one of those bind
       level: debug2
       format: "[%(asctime)s.%(msecs)03d] %(levelname)s: %(check_info)s: %(message)s"
       append: false
+
+
+.. _builtin-configuration:
+
+Builtin configuration
+=====================
+
+This is the builtin configuration that ReFrame always loads.
+
+.. literalinclude:: ../reframe/core/settings.py
+   :lines: 6-
+
+.. seealso::
+
+   See also how configuration files are :ref:`loaded <manpage-configuration>` and how you can specify them with the :option:`--config-file` option.

docs/manpage.rst

@@ -1115,7 +1115,7 @@ Miscellaneous options
    Use ``FILE`` as configuration file for ReFrame.
 
    This option can be passed multiple times, in which case multiple configuration files will be read and loaded successively.
-   The base of the configuration chain is always the builtin configuration file, namely the ``${RFM_INSTALL_PREFIX}/reframe/core/settings.py``.
+   The base of the configuration chain is always the :ref:`builtin configuration file <builtin-configuration>`, namely the ``${RFM_INSTALL_PREFIX}/reframe/core/settings.py``.
    At any point, the user can "break" the chain of configuration files by prefixing the configuration file name with a colon as in the following example: ``-C :/path/to/new_config.py``.
    This will ignore any previously loaded configuration file and will only load the one specified.
    Note, however, that the builtin configuration file cannot be overriden;