[Messenger] Add Supervisor stopwaitsecs example code

Co-authored-by: Oskar Stark <oskarstark@googlemail.com>

Author: AymDev

messenger.rst

@@ -666,6 +666,11 @@ current message before exiting, as long as the `PCNTL`_ extension is installed.
     In these cases you need to add a ``stopwaitsecs`` key to the program
     configuration (with a value of the desired grace period in seconds)
     in order to perform a graceful shutdown.
+    
+    .. code-block:: ini
+    
+        [program:x]
+        stopwaitsecs=20
 
 .. _messenger-retries-failures: