[3.13] gh-141004: Document C APIs for dictionary keys, values, and items (GH-141009) (GH-141513)

gh-141004: Document C APIs for dictionary keys, values, and items (GH-141009)
(cherry picked from commit f72768f30e6ed9253eb3b6374b4395dfcaf4842a)

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