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

[Martin Panter]

Martin Panter added the comment:

setattr.3.patch changes to documenting setting as the main purpose, but mentions deleting also works but is deprecated in favour of the main deletion functions. I also clarified that the slots have to support deleting via NULL.

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Added file: http://bugs.python.org/file41259/setattr.3.patch

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