gh-141004: Document PyOS_mystr(n)icmp

[StanFromIreland]

• Issue: Document the entire C API #141004

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📚 Documentation preview 📚: https://cpython-previews--141760.org.readthedocs.build/

[encukou]

These are aliases of PyOS_stricmp/PyOS_strnicmp, except on Windows (where the my-less variants are aliases of stricmp/strnicmp -- I assume that's because we know they work on that platform).
Could you document them in c-api/conversion?

Ideally also mention that they only ignore case of ASCII characters, and aren't affected by locale.

[vstinner]

LGTM