bpo-43957: [Enum] Deprecate ``TypeError`` from containment checks. (GH-25670)

In 3.12 ``True`` or ``False`` will be returned for all containment checks,
with ``True`` being returned if the value is either a member of that enum
or one of its members' value.
diff --git a/Doc/library/enum.rst b/Doc/library/enum.rst
index 91c214e..b5f9c2f 100644
--- a/Doc/library/enum.rst
+++ b/Doc/library/enum.rst
@@ -140,6 +140,12 @@
         >>> some_var in Color
         True
 
+      .. note::
+
+         In Python 3.12 it will be possible to check for member values and not
+         just members; until then, a ``TypeError`` will be raised if a
+         non-Enum-member is used in a containment check.
+
    .. method:: EnumType.__dir__(cls)
 
       Returns ``['__class__', '__doc__', '__members__', '__module__']`` and the