[3.13] Docs C API: Clarify what happens when null bytes are passed to `PyUnicode_AsUTF8` (GH-127458) (#129080)

Docs C API: Clarify what happens when null bytes are passed to `PyUnicode_AsUTF8` (GH-127458)
(cherry picked from commit e792f4bc2e712bb6e2143599d2b88dd339de83e6)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Stan U. <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Tomas R. <tomas.roun8@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>