[docs] Add documentation for typing.Generator (issue 27689)

[levkivskyi at gmail.com]

http://bugs.python.org/review/27689/diff/18074/Doc/library/typing.rst
File Doc/library/typing.rst (right):

http://bugs.python.org/review/27689/diff/18074/Doc/library/typing.rst#newcode474
Doc/library/typing.rst:474: while sent is not None:
One more comment: One might be confused why send type is float and not
Optional[float], maybe you could add an explanation that one anyway can
only send None as a first value, so that Optional is redundant.

http://bugs.python.org/review/27689/