Expand on docs around event priorities.

Add some detail around how priorities are handled.

src/Event/EventManager.php

@@ -89,8 +89,8 @@ public static function instance($manager = null) {
  * is an instance of Cake\Event\EventListener this argument will be ignored
  *
  * @param array $options used to set the `priority` flag to the listener. In the future more options may be added.
- * Priorities are handled like queues, and multiple attachments added to the same priority queue will be treated in
- * the order of insertion.
+ * Priorities are treated as queues. Lower values are called before higher ones, and multiple attachments
+ * added to the same priority queue will be treated in the order of insertion.
  *
  * @return void
  * @throws \InvalidArgumentException When event key is missing or callable is not an