[3.13] document `Py_VISIT` as a macro in the docs (GH-133688) (#134187)

document `Py_VISIT` as a macro in the docs (GH-133688)
(cherry picked from commit bb32f3c698f5192dddd2d2f33c7c0a3d05afc223)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>