bpo-40721: add note about enum member name case (GH-22231)

* UPPER_CASE preferred as enum members are constants
(cherry picked from commit 542e1df2b0)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

Doc/library/enum.rst

@@ -19,6 +19,12 @@ An enumeration is a set of symbolic names (members) bound to unique,
 constant values.  Within an enumeration, the members can be compared
 by identity, and the enumeration itself can be iterated over.
 
+.. note:: Case of Enum Members
+
+    Because Enums are used to represent constants we recommend using
+    UPPER_CASE names for enum members, and will be using that style
+    in our examples.
+
 
 Module Contents
 ---------------