[docs] Document that tp_setattro and tp_setattr are used for deleting attributes (issue 25701)

[victor.stinner at gmail.com]

http://bugs.python.org/review/25701/diff/16034/Doc/c-api/object.rst
File Doc/c-api/object.rst (right):

http://bugs.python.org/review/25701/diff/16034/Doc/c-api/object.rst#newcode72
Doc/c-api/object.rst:72: otherwise set it to *v* (``o.attr_name = v``).
Returns ``-1`` on failure.
I know that it's not a common Python doc, but I prefer to explicitly say
that an exception is raised in case of error. I suggest:

Raise an exception and returns -1 on failure, return 0 on success.

(Same change for other functions below)

http://bugs.python.org/review/25701/diff/16034/Include/abstract.h
File Include/abstract.h (right):

http://bugs.python.org/review/25701/diff/16034/Include/abstract.h#newcode196
Include/abstract.h:196: (o.attr_name=v). Returns -1 on failure.
Raise an exception and returns -1 on failure, return 0 on success.

http://bugs.python.org/review/25701/