doc: improve configuration docs

Initial proposal to improve some of Flask-RESTX' configuration options.

Author: ziirish

doc/configuration.rst

@@ -13,14 +13,37 @@ Flask-RESTX provides the following `Flask configuration values <https://flask.pa
 
 .. py:data:: RESTX_VALIDATE
 
+   Whether to enforce payload validation by default when using the
+   ``@api.expect()`` decorator. See the `@api.expect()
+   <swagger.html#the-api-expect-decorator>`__ documentation for details.
+   This setting defaults to ``False``.
+
 .. py:data:: RESTX_MASK_HEADER
 
+  Choose the name of the *Header* that will contain the masks to apply to your
+  answer. See the `Fields masks <mask.html>`__ documentation for details.
+  This setting defaults to ``X-Fields``.
+
 .. py:data:: RESTX_MASK_SWAGGER
 
+  Whether to enable the mask documentation in your swagger or not. See the
+  `mask usage <mask.html#usage>`__ documentation for details.
+  This setting defaults to ``True``.
+
 .. py:data:: RESTX_INCLUDE_ALL_MODELS
 
+  This option allows you to include all defined models in the generated Swagger
+  documentation, even if they are not explicitly used in either ``expect`` nor
+  ``marshal_with`` decorators.
+  This setting defaults to ``False``.
+
 .. py:data:: BUNDLE_ERRORS
 
+  Bundle all the validation errors instead of returning only the first one
+  encountered. See the `Error Handling <parsing.html#error-handling>`__ section
+  of the documentation for details.
+  This setting defaults to ``False``.
+
 .. py:data:: ERROR_404_HELP
 
 .. py:data:: HTTP_BASIC_AUTH_REALM
@@ -39,4 +62,4 @@ Flask-RESTX provides the following `Flask configuration values <https://flask.pa
 
 .. py:data:: SWAGGER_UI_OAUTH_REALM
 
-.. py:data:: SWAGGER_SUPPORTED_SUBMIT_METHODS
+.. py:data:: SWAGGER_SUPPORTED_SUBMIT_METHODS