[3.12] gh-89414: Document that SIGCLD is not available on macOS (GH-113580) (#113585)

gh-89414: Document that SIGCLD is not available on macOS (GH-113580)

Document that SIGCLD is not available on macOS
(cherry picked from commit f48a1bc)

Co-authored-by: Ronald Oussoren <[email protected]>

Author: miss-islington

Doc/library/signal.rst

@@ -157,6 +157,8 @@ The variables defined in the :mod:`signal` module are:
 
    Alias to :data:`SIGCHLD`.
 
+   .. availability:: not macOS.
+
 .. data:: SIGCONT
 
    Continue the process if it is currently stopped