[docs] Missing information in the C api docs

[Alexander Kauer]

Hello everybody,

recently I embedded Python into one of my applications, which included
some string parsing.
In my opinion the doc page about parsing arguments
(http://docs.python.org/c-api/arg.html) is missing the information, that
you must not free the returned const char *, like e.g. said here
(http://docs.python.org/c-api/string.html#PyString_AsString).
It was quite confusing for my first, so I would suggest adding this tiny
bit of information.

Have a nice day
Alexander Kauer

ps: I am not subscripted to the mailing list, so if you want to contact
me use my email (alexanderkauer+python at gmail.com)