feat: Add downloadcompleted event description (#7615)

Missing piece of #7609

Author: tykus160

lib/player.js

@@ -105,6 +105,17 @@ goog.requireType('shaka.media.PresentationTimeline');
  */
 
 
+/**
+ * @event shaka.Player.DownloadCompleted
+ * @description Fired when a download has completed.
+ * @property {string} type
+ *   'downloadcompleted'
+ * @property {!shaka.extern.Request} request
+ * @property {!shaka.extern.Response} response
+ * @exportDoc
+ */
+
+
 /**
  * @event shaka.Player.DownloadFailed
  * @description Fired when a download has failed, for any reason.