[3.14] gh-141004: Document `Py_DTSF_*` macros (GH-141310) (GH-141329)

gh-141004: Document `Py_DTSF_*` macros (GH-141310)
(cherry picked from commit 9b0179fa87fee39df0f75bd84fc2dd75f1d00553)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>