Document Py_ssize_t. (GH-92512)

It fixes 252 errors from a Sphinx nitpicky run (sphinx-build -n). But
there's 8182 errors left.

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>
(cherry picked from commit 664aa94b570a4a8f3535efb2e3d638a4ab655943)

Co-authored-by: Julien Palard <julien@palard.fr>