Add xmldoc to MigratableStorage

osu.Game/IO/MigratableStorage.cs

@@ -9,6 +9,9 @@ using osu.Framework.Platform;
 
 namespace osu.Game.IO
 {
+    /// <summary>
+    /// A <see cref="WrappedStorage"/> that is migratable to different locations. 
+    /// </summary>
     public abstract class MigratableStorage : WrappedStorage
     {
         internal static readonly string[] IGNORE_DIRECTORIES = { "cache" };