[issue41224] Document is_annotate() in symtable and update doc strings

[Andrés Delfino]

Change by Andrés Delfino <adelfino at gmail.com>:


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nosy: +adelfino
nosy_count: 2.0 -> 3.0
pull_requests: +22838
pull_request: https://github.com/python/cpython/pull/23861

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<https://bugs.python.org/issue41224>
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