Address PR feeback

Author: arcadiogarcia

Microsoft.Toolkit.Uwp.UI.Controls.Core/DropShadowPanel/DropShadowPanel.cs

@@ -167,6 +167,8 @@ private void UpdateShadowMask()
             {
                 CompositionBrush mask = null;
 
+                // We check for IAlphaMaskProvider first, to ensure that we use the custom
+                // alpha mask even if Content happens to extend any of the other classes
                 if (Content is IAlphaMaskProvider maskedControl)
                 {
                     mask = maskedControl.GetAlphaMask();