[docs] [issue16574] clarify policy on updates to final peps

[Roundup Robot]

Roundup Robot added the comment:

New changeset 0c4006b7c7ff by Berker Peksag in branch 'default':
Issue #16574: Clarify that once a PEP has implemented, it needs be
https://hg.python.org/devguide/rev/0c4006b7c7ff

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nosy: +python-dev

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