[docs] Expand documentation about type aliases and NewType in the typing module (issue 27644)

[levkivskyi at gmail.com]

I have only one optional comment.


http://bugs.python.org/review/27644/diff/18014/Doc/library/typing.rst
File Doc/library/typing.rst (right):

http://bugs.python.org/review/27644/diff/18014/Doc/library/typing.rst#newcode103
Doc/library/typing.rst:103: is not possible to create another
``NewType`` based on a ``Derived`` type::
I don't think we need to mention this limitation. It might be useful to
further derive the type returned by NewType. The PEP says that Base must
be a proper class, but I could imagine that this limitation will be
relaxed slightly to allow also unique types returned by NewType.

However, if it is OK with Guido, then it is also OK with me.

http://bugs.python.org/review/27644/