[docs] [issue33014] Clarify str.isidentifier docstring; fix keyword.iskeyword docstring

[Sanyam Khurana]

Change by Sanyam Khurana <sanyam.khurana01 at gmail.com>:


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keywords: +patch
pull_requests: +5850
stage: needs patch -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33014>
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