[3.13] gh-133264: Correct documentation of how Py_Main and Py_RunMain handle SystemExit (GH-135337) (#135395)

gh-133264: Correct documentation of how Py_Main and Py_RunMain handle SystemExit (GH-135337)
(cherry picked from commit b706ff003c536c5bca24dfdd3a8917bffcfa3df1)

Co-authored-by: Malcolm Smith <smith@chaquo.com>