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

[Terry J. Reedy]

Change by Terry J. Reedy <tjreedy at udel.edu>:


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versions: +Python 2.7, Python 3.6, Python 3.8

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