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

[Serhiy Storchaka]

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).

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25701>
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