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_pydoc.py

@@ -453,7 +453,7 @@ class PydocDocTest(unittest.TestCase):
             zero = 0
             one = 1
         doc = pydoc.render_doc(BinaryInteger)
-        self.assertIn('<BinaryInteger.zero: 0>', doc)
+        self.assertIn('BinaryInteger.zero', doc)
 
     def test_mixed_case_module_names_are_lower_cased(self):
         # issue16484