gh-91755: Document Py_IncRef and Py_DecRef as C functions (GH-91805)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 58a3d28039863b014f57a1ac93b51e20920ebe7b)

Co-authored-by: Charlie Zhao <zhaoyu_hit@qq.com>