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

[ghost.adh at runbox.com]

https://bugs.python.org/review/27242/diff/17812/Doc/library/constants.rst
File Doc/library/constants.rst (right):

https://bugs.python.org/review/27242/diff/17812/Doc/library/constants.rst#newcode38
Doc/library/constants.rst:38: When ``NotImplemented`` is returned, the
interpreter will then try the
Please, wrap everything to 80 columns.

https://bugs.python.org/review/27242/diff/17812/Doc/library/constants.rst#newcode44
Doc/library/constants.rst:44: :meth:`__lt__` or :meth:`__add__`. The
appropriate way to signal that an
Sentences should be separated by two spaces. Here and everywhere below.

https://bugs.python.org/review/27242/diff/17812/Doc/library/constants.rst#newcode47
Doc/library/constants.rst:47: calling the superclass' method with
:func:`super`) or raising an exception
superclass's

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