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/test/test_signal.py

@@ -872,7 +872,7 @@ class PendingSignalsTests(unittest.TestCase):
 
         %s
 
-        blocked = %s
+        blocked = %r
         signum = signal.SIGALRM
 
         # child: block and wait the signal