[docs] Document auto __ne__ generation; provide a use case for non-trivial __ne__ (issue 4395)

[storchaka at gmail.com]

http://bugs.python.org/review/4395/diff/15237/Doc/reference/datamodel.rst
File Doc/reference/datamodel.rst (right):

http://bugs.python.org/review/4395/diff/15237/Doc/reference/datamodel.rst#newcode1280
Doc/reference/datamodel.rst:1280: inverts the result.  There are no
other implied relationships
Is it worth explicitly mention that __ne__ inverts the result if it is
not NotImplemented?

http://bugs.python.org/review/4395/diff/15237/Lib/test/test_binop.py
File Lib/test/test_binop.py (right):

http://bugs.python.org/review/4395/diff/15237/Lib/test/test_binop.py#newcode387
Lib/test/test_binop.py:387: self.assertTrue(issubclass(V, B))
assertIsSubclass?

http://bugs.python.org/review/4395/