bpo-40066: Enum: modify repr() and str() (GH-22392)

* Enum: streamline repr() and str(); improve docs

- repr() is now ``enum_class.member_name``
- stdlib global enums are ``module_name.member_name``
- str() is now ``member_name``
- add HOW-TO section for ``Enum``
- change main documentation to be an API reference

Doc/howto/index.rst

@@ -17,6 +17,7 @@ Currently, the HOWTOs are:
    cporting.rst
    curses.rst
    descriptor.rst
+   enum.rst
    functional.rst
    logging.rst
    logging-cookbook.rst