[3.14] gh-135755: Docs: C API: Document missing `PyFunction_GET*` macros (GH-135762) (GH-135916)

gh-135755: Docs: C API: Document missing `PyFunction_GET*` macros (GH-135762)

(cherry picked from commit ca87a47b3d92aabaefbbe79c0493d66602184b41)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Brian Schubert <brianm.schubert@gmail.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>