Added event documentation to keydown:delete.

src/deleteobserver.js

@@ -77,3 +77,13 @@ export default class DeleteObserver extends Observer {
  * @param {Number} data.sequence A number describing which subsequent delete event it is without the key being released.
  * If it's 2 or more it means that the key was pressed and hold.
  */
+
+/**
+ * Fired when a keydown event was handled by {@link module:typing/deleteobserver~DeleteObserver}.
+ * It can be used to determine if the event was handled by {@link module:typing/deleteobserver~DeleteObserver} or it is
+ * a generic event created by {@link module:engine/view/observer/keyobserver~KeyObserver}.
+ *
+ * @see module:engine/view/observer/keyobserver~KeyObserver
+ * @event module:engine/view/document~Document#event:keydown:delete
+ * @param {module:engine/view/observer/keyobserver~KeyEventData} keyEventData
+ */