[issue1944] Document PyUnicode_* API

[Furkan Onder]

Change by Furkan Onder <furkanonder at protonmail.com>:


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nosy: +furkanonder
nosy_count: 8.0 -> 9.0
pull_requests: +19322
stage: needs patch -> patch review
pull_request: https://github.com/python/cpython/pull/20011

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue1944>
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