[docs] [issue36404] Document PendingDeprecationWarning as deprecated

[Inada Naoki]

Change by Inada Naoki <songofacandy at gmail.com>:


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assignee:  -> docs at python
components: +Documentation
nosy: +docs at python

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36404>
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