Add doc-comment for IBackbufferProvider

osu.Framework/Graphics/Containers/IBackbufferProvider.cs

@@ -5,6 +5,9 @@
 
 namespace osu.Framework.Graphics.Containers
 {
+    /// <summary>
+    /// A container which ensures that its children are drawn to a framebuffer.
+    /// </summary>
     [Cached]
     public interface IBackbufferProvider : IContainer
     {