Add messenger.stop_worker_on_signals option documentation

Author: Rokas Mikalkėnas

messenger.rst

@@ -757,8 +757,12 @@ Graceful Shutdown
 .................
 
 If you install the `PCNTL`_ PHP extension in your project, workers will handle
-the ``SIGTERM`` POSIX signal to finish processing their current message before
-terminating.
+the ``SIGTERM`` or ``SIGINT`` POSIX signals to finish processing their current
+message before terminating.
+
+However, you might prefer to use different POSIX signals for graceful shutdown.
+You can override default ones by setting ``framework.messenger.stop_worker_on_signals``
+configuration option.
 
 In some cases the ``SIGTERM`` signal is sent by Supervisor itself (e.g. stopping
 a Docker container having Supervisor as its entrypoint). In these cases you