improve a comment

gevent/callbacks.c

@@ -116,7 +116,8 @@ static void gevent_callback(struct PyGeventLoopObject* loop, PyObject* callback,
         }
     }
     if (!ev_is_active(c_watcher)) {
-        /* watcher will never be run again: calling stop() will clear 'callback' and 'args' */
+        /* watcher will never be run again: calling stop() will clear 'callback' and 'args'
+         * also it will release the reference held by watcher to itself and does ref() if unref() was done before */
         gevent_stop(watcher, loop);
     }
 end: