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

[Roundup Robot]

Roundup Robot added the comment:

New changeset 50711c80ff76 by Martin Panter in branch '3.5':
Issue #25701: Document C API functions that both set and delete objects
https://hg.python.org/cpython/rev/50711c80ff76

New changeset 7bb7173cd97a by Martin Panter in branch 'default':
Issue #25701: Merge set and delete documentation from 3.5
https://hg.python.org/cpython/rev/7bb7173cd97a

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nosy: +python-dev

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