[3.13] gh-146318: Document that signal.SIGSTOP is Unix-only (GH-146319) (#146469)

gh-146318: Document that signal.SIGSTOP is Unix-only (GH-146319)
(cherry picked from commit e44993a)

Co-authored-by: Jonathan Dung <jonathandung@yahoo.com>

Author: miss-islington

Doc/library/signal.rst

@@ -230,6 +230,8 @@ The variables defined in the :mod:`signal` module are:
 
    Stop executing (cannot be caught or ignored).
 
+   .. availability:: Unix.
+
 .. data:: SIGSTKFLT
 
    Stack fault on coprocessor. The Linux kernel does not raise this signal: it