Added a comment about RenderLoop threading.

src/Avalonia.SceneGraph/Rendering/IRenderLoop.cs

@@ -10,6 +10,10 @@ namespace Avalonia.Rendering
         /// <summary>
         /// Raised when the render loop ticks to signal a new frame should be drawn.
         /// </summary>
+        /// <remarks>
+        /// This event can be raised on any thread; it is the responsibility of the subscriber to
+        /// switch execution to the right thread.
+        /// </remarks>
         event EventHandler<EventArgs> Tick;
     }
 }