[issue42048] Document Argument Clinic's defining_class converter

[Erlend Egeberg Aasland]

Change by Erlend Egeberg Aasland <erlend.aasland at innova.no>:


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keywords: +patch
pull_requests: +22823
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/23978

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42048>
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