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

Lib/inspect.py

@@ -2455,9 +2455,6 @@ class _ParameterKind(enum.IntEnum):
     KEYWORD_ONLY = 3
     VAR_KEYWORD = 4
 
-    def __str__(self):
-        return self._name_
-
     @property
     def description(self):
         return _PARAM_NAME_MAPPING[self]