Add and change xmldocs

osu.Game/Beatmaps/IWorkingBeatmap.cs

@@ -42,7 +42,7 @@ public interface IWorkingBeatmap
         Storyboard Storyboard { get; }
 
         /// <summary>
-        /// Retrieves the <see cref="Skin"/> which this <see cref="WorkingBeatmap"/> provides.
+        /// Retrieves the <see cref="IBeatmapSkin"/> which this <see cref="WorkingBeatmap"/> provides.
         /// </summary>
         IBeatmapSkin Skin { get; }
 

osu.Game/Skinning/IBeatmapSkin.cs

@@ -3,6 +3,9 @@
 
 namespace osu.Game.Skinning
 {
+    /// <summary>
+    /// Marker interface for skins that originate from beatmaps.
+    /// </summary>
     public interface IBeatmapSkin : ISkin
     {
     }