[docs] [issue25701] Document that tp_setattro and tp_setattr are used for deleting attributes

New submission from Serhiy Storchaka: tp_setattro and tp_setattr fields of PyTypeObject are used for deleting attributes. In this case the third argument is NULL. This should be documented. Not awareness of this can cause a segmentation fail (for example see issue25691). ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue25701> _______________________________________