[docs] Code, test, and doc review for PEP-0435 Enum (issue 17947)

[ethan at stoneleaf.us]

http://bugs.python.org/review/17947/diff/8339/Doc/library/enum.rst
File Doc/library/enum.rst (right):

http://bugs.python.org/review/17947/diff/8339/Doc/library/enum.rst#newcode292
Doc/library/enum.rst:292: 
Added:

.. warning::

    In order to support the singleton nature of enumeration members,
pickle
    protocol version 2 or higher must be used.

http://bugs.python.org/review/17947/