gh-107298: Fix references to undocumented APIs

[vstinner]

• Issue: Fix Sphinx warnings in the C API documentation #107298

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📚 Documentation preview 📚: https://cpython-previews--107304.org.readthedocs.build/

[vstinner]

@serhiy-storchaka: Ok, here is a more controversial change: fix references to undocumented APIs, but don't attempt to document them.

Sorry, I'm only motived to fix warnings, to make reviews more pleasant. I'm not motivated to write more documentation.

So what do you think? Is it ok do that? For now, I mark this change as a draft.

[serhiy-storchaka]

We should not sweep problems under the carpet. Warnings serve as reminders of lacks in the documentation.

If you want to filter out undocumented symbols from other issues, it is better to add them in the nitpick_ignore list. At least we will have a list of problems to solve in one place and can work on them one by one.

[vstinner]

Ok, I will try to do that. I abandon this approach.