Document extend_schema_view support for @action (#1178)

* Document extend_schema_view support for @action

* Fix formatting

* Update customization.rst

fix rst syntax

* Update utils.py

* Update utils.py

oh boy

---------

Co-authored-by: T. Franzel <tfranzel@users.noreply.github.com>

Author: johnthagen

docs/customization.rst

@@ -74,6 +74,18 @@ discovered in the introspection.
         class XViewset(mixins.ListModelMixin, viewsets.GenericViewSet):
             ...
 
+  This also supports annotating extra user-defined DRF ``@action``\s
+
+  .. code-block:: python
+
+        @extend_schema_view(
+            notes=extend_schema(description='text')
+        )
+        class XViewset(mixins.ListModelMixin, viewsets.GenericViewSet):
+            @action(detail=False)
+            def notes(self, request):
+                ...
+
 .. note:: You may also use :py:func:`@extend_schema <drf_spectacular.utils.extend_schema>` on views
   to attach annotations to all methods in that view (e.g. tags). Method annotations will take precedence
   over view annotation.

drf_spectacular/utils.py

@@ -644,6 +644,8 @@ def extend_schema_view(**kwargs) -> Callable[[F], F]:
     This decorator also takes care of safely attaching annotations to derived view methods,
     preventing leakage into unrelated views.
 
+    This decorator also supports custom DRF ``@action`` with the method name as the key.
+
     :param kwargs: method names as argument names and :func:`@extend_schema <.extend_schema>`
       calls as values
     """