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

[Roundup Robot]

Roundup Robot added the comment:

New changeset 83e3c863594c by Martin Panter in branch '2.7':
Issue #25701: Document that some C APIs can both set and delete items
https://hg.python.org/cpython/rev/83e3c863594c

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