[3.13] gh-139843: Document signals (SIGSTOP, SIGVTALRM, SIGPROF) to fix sphinx references (GH-139896) (GH-139909)

gh-139843: Document signals (SIGSTOP, SIGVTALRM, SIGPROF) to fix sphinx references (GH-139896)
(cherry picked from commit 302f19d1f1880dd29deb5ad02fa2c21639319559)

Co-authored-by: Weilin Du <108666168+LamentXU123@users.noreply.github.com>