[3.12] gh-102468: Document `PyCFunction_New*` and `PyCMethod_New` (GH-112557) (GH-114119)

gh-102468: Document `PyCFunction_New*` and `PyCMethod_New` (GH-112557)
(cherry picked from commit a482bc67ee786e60937a547776fcf9528810e1ce)

Co-authored-by: AN Long <aisk@users.noreply.github.com>
Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>