[DI][EventDispatcher] added contract for implementation

fixes #36708.

Author: Matthias Derer

EventSubscriberInterface.php

@@ -40,6 +40,9 @@ interface EventSubscriberInterface
      *  * ['eventName' => ['methodName', $priority]]
      *  * ['eventName' => [['methodName1', $priority], ['methodName2']]]
      *
+     * The code must not depend on runtime state as it will only be called at compile time.
+     * All logic depending on runtime state must be put into the individual methods handling the events.
+     *
      * @return array The event names to listen to
      */
     public static function getSubscribedEvents();