[docs] Improve doc for str(bytesobject) (issue 13538)

[chris.jerdonek at gmail.com]

http://bugs.python.org/review/13538/diff/6626/Doc/library/functions.rst
File Doc/library/functions.rst (right):

http://bugs.python.org/review/13538/diff/6626/Doc/library/functions.rst#newcode1264
Doc/library/functions.rst:1264: :meth:`object.__str__()
<object.__str__>`, which is the "informal" or nicely
I am writing out "object.__str__()" explicitly because it is important
that the rendered version show the invocation of the method.  Strictly
speaking, while Sphinx will render :meth:`object.__str__` the same way,
I want to make it clear that the final rendering matters in this case.

http://bugs.python.org/review/13538/