[docs] document PEP 448 (issue 24136)

[vadmium+py at gmail.com]

https://bugs.python.org/review/24136/diff/17575/Doc/reference/expressions.rst
File Doc/reference/expressions.rst (right):

https://bugs.python.org/review/24136/diff/17575/Doc/reference/expressions.rst#newcode653
Doc/reference/expressions.rst:653: :   ("," `keyword_argument` |
`starred_argument`)*
There should probably be a comma before starred_(keyword_)argument. What
you did for positional_arguments below should work, although I think it
might read better expanded:

("," `keyword_argument` | "," `starred_argument`)*

https://bugs.python.org/review/24136/diff/17575/Doc/reference/expressions.rst#newcode655
Doc/reference/expressions.rst:655: positional_arguments:
(`positional_argument` | `starred_argument`)
positional_argument (singular) seems to be missing

https://bugs.python.org/review/24136/diff/17575/Doc/reference/expressions.rst#newcode664
Doc/reference/expressions.rst:664: Keyword arguments must follow
positional arguments, and ``**`` unpackings must follow ``*``
unpackings::
This rule might go better further down, where it discusses mixing
keyword arguments and starred arguments. Also, it would be good to
word-wrap to two lines.

https://bugs.python.org/review/24136/diff/17575/Doc/reference/expressions.rst#newcode672
Doc/reference/expressions.rst:672: See :pep:`448` for more details about
unpacking generalizations.
It would be good to include any missing details in the main
documentation (but a link is still good as a reference, and is better
than nothing :)

https://bugs.python.org/review/24136/