[issue29086] Document C API that is not part of the limited API

[Roundup Robot]

Change by Roundup Robot <devnull at psf.upfronthosting.co.za>:


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nosy: +python-dev
nosy_count: 3.0 -> 4.0
pull_requests: +22089
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23185

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue29086>
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