style(event): added docblock return types as per the CR

Refs phalcon#17

Author: niden

Diff for: src/Events/Event.php

@@ -123,6 +123,8 @@ public function getType(): string
      *     $event->stop();
      * }
      * ```
+     *
+     * @return bool
      */
     public function isCancelable(): bool
     {
@@ -131,6 +133,8 @@ public function isCancelable(): bool
 
     /**
      * Check whether the event is currently stopped.
+     *
+     * @return bool
      */
     public function isStopped(): bool
     {