gmain: Add a clarifying comment about exit statuses vs wait statuses

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2216

(cherry-picked from commit 7b93693ab3007670a3d95d6ac3cb9260c5643493)

glib/gmain.c

@@ -342,6 +342,8 @@ struct _GChildWatchSource
 {
   GSource     source;
   GPid        pid;
+  /* On Unix this is a wait status, which is the thing you pass to WEXITSTATUS()
+   * to get the status returned from the process’ main() or passed to exit(): */
   gint        child_status;
 #ifdef G_OS_WIN32
   GPollFD     poll;