bpo-43162: [Enum] update docs, renable doc tests (GH-24487)

* update docs, renable doc tests
* make deprecation warning active for two releases
diff --git a/Lib/enum.py b/Lib/enum.py
index 55299c5..84c7b0d 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -148,7 +148,7 @@ def __get__(self, instance, ownerclass=None):
                     import warnings
                     warnings.warn(
                             "accessing one member from another is not supported, "
-                            " and will be disabled in 3.11",
+                            " and will be disabled in 3.12",
                             DeprecationWarning,
                             stacklevel=2,
                             )