Document PlaybackStartEventArgs

MediaBrowser.Controller/Library/PlaybackStartEventArgs.cs

@@ -1,5 +1,8 @@
 namespace MediaBrowser.Controller.Library
 {
+    /// <summary>
+    /// An event that occurs when playback is started.
+    /// </summary>
     public class PlaybackStartEventArgs : PlaybackProgressEventArgs
     {
     }