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

Petr Viktorin added the comment: The bigger problem is that currently, it's not clear what is limited API and what is not. I am working to propose a PEP to fix this. It will also cover documentation: https://discuss.python.org/t/pre-pep-maintaining-the-stable-abi/6986 ---------- nosy: +petr.viktorin _______________________________________ Python tracker https://bugs.python.org/issue29086 _______________________________________