[SPIGOT-950] Make handlerList requirement really obvious.

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/event/Event.java

@@ -5,6 +5,8 @@ import org.bukkit.plugin.PluginManager;
 /**
  * Represents an event.
  *
+ * All events require a static method named getHandlerList() which returns the same {@link HandlerList} as {@link #getHandlers()}.
+ *
  * @see PluginManager#callEvent(Event)
  * @see PluginManager#registerEvents(Listener,Plugin)
  */