gh-93464: [Enum] Add versionchanged tag (GH-99997)

(cherry picked from commit e3a3863)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>
Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>

Author: 3 people

Doc/library/enum.rst

@@ -811,6 +811,11 @@ Utilities and Decorators
       * ``THREE = [auto(), -3]`` will *not* work (``<auto instance>, -3`` is used to
         create the ``THREE`` enum member)
 
+   .. versionchanged:: 3.11.1
+
+      In prior versions, ``auto()`` had to be the only thing
+      on the assignment line to work properly.
+
    ``_generate_next_value_`` can be overridden to customize the values used by
    *auto*.