[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#newcode1268
Doc/library/functions.rst:1268: representation :func:`repr` of *object*.
Incidentally, after discovering recently that the object type defines a
default implementation of object.__str__(), it's not clear to me when
this case can even arise.  Do you know?

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