[docs] add a "class str" entry to the docs (issue 16209)

[ezio.melotti at gmail.com]

http://bugs.python.org/review/16209/diff/6699/Doc/library/functions.rst
File Doc/library/functions.rst (right):

http://bugs.python.org/review/16209/diff/6699/Doc/library/functions.rst#newcode1244
Doc/library/functions.rst:1244: Return a :class:`str` version of
*object*.  See :func:`str` for details.
Wouldn't :func:`str` link to this entry?  Or would it just link to
:class:`str` but add the () at the end?

http://bugs.python.org/review/16209/