[issue41430] Document C docstring behavior

[James Corbett]

Change by James Corbett <james.h.corbett at gmail.com>:


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keywords: +patch
pull_requests: +20816
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/21673

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue41430>
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