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

Serhiy Storchaka <storchaka+cpython@gmail.com> added the comment: I have found that there is an option :stableabi: for C API elements. But it is not used in the documentation. Shouldn't we start to use it? ---------- _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue29086> _______________________________________