Make ETSConfig class documentation visible in the API docs (#1688)

This PR provides an alternative to #1669. The goal is to make the ETSConfig docstrings available in the API docs, so that a search for "ETSConfig" in the online documentation gives useful results.

To that end, this PR:

* Renames the ETSConfig class to ETSConfigType
* Keeps the ETSConfig instance the same as before (previously, the created instance shadowed the class)
* Updates the main docstrings of ETSConfigType and ETSConfig.

There shouldn't be backwards-compatibility concerns with the rename: the ETSConfig class wasn't available before (since it was shadowed by the instance), and the trick of using type(ETSConfig) to get the class will still work.

I'm deliberately not exposing ETSConfigType in traits.api, since in most circumstances it shouldn't be used directly

docs/source/traits_api_reference/traits.etsconfig.rst

@@ -10,4 +10,6 @@
 .. automodule:: traits.etsconfig.etsconfig
     :no-members:
 
+.. autoclass:: ETSConfigType
+
 .. autodata:: ETSConfig

traits/etsconfig/etsconfig.py

@@ -41,14 +41,20 @@ def __init__(self, message="", toolkit=None, *args):
         self.args = args
 
 
-class ETSConfig(object):
+class ETSConfigType:
     """
     Enthought Tool Suite configuration information.
 
-    This class should not use ANY other package in the tool suite so that it
-    will always work no matter which other packages are present.
+    Instances of this class record state useful for ETS-using applications,
+    including the current GUI toolkit in use, and data and home directory
+    setttings.
 
+    Users typically shouldn't make use of this class directly. Instead, use the
+    module-level :data:`~.ETSConfig` instance of this class, which is shared
+    between the various ETS packages.
     """
+    # This class should not use ANY other package in the tool suite so that it
+    # will always work no matter which other packages are present.
 
     ###########################################################################
     # 'object' interface.
@@ -511,7 +517,10 @@ def _initialize_user_data(self):
         return usr_dir
 
 
-# We very purposefully only have one object and do not export the class. We
-# could have just made everything class methods, but that always seems a bit
-# gorpy, especially with properties etc.
-ETSConfig = ETSConfig()
+#: This single instance of :class:`~.ETSConfigType` is shared between the
+#: various ETS packages, and used to store global state relevant to
+#: ETS-using applications.
+#:
+#: See https://github.com/enthought/traits/discussions/1666 for a discussion
+#: of writing tests that depend on :data:`~.ETSConfig` state.
+ETSConfig = ETSConfigType()