[docs] Clarify the use cases of NotImplemented in the docs (issue 27242)

[ethan at stoneleaf.us]

http://bugs.python.org/review/27242/diff/17555/Doc/library/constants.rst
File Doc/library/constants.rst (right):

http://bugs.python.org/review/27242/diff/17555/Doc/library/constants.rst#newcode48
Doc/library/constants.rst:48: is the encouraged way to do this.
Still not sure of above section.  Asked about it on Python-Dev.

http://bugs.python.org/review/27242/diff/17555/Doc/library/exceptions.rst
File Doc/library/exceptions.rst (right):

http://bugs.python.org/review/27242/diff/17555/Doc/library/exceptions.rst#newcode246
Doc/library/exceptions.rst:246: the requested operation, and that it
should try the reflected version.
of the other operand

http://bugs.python.org/review/27242/