update comment for ResolveByName attribute.

src/Avalonia.Controls/ResolveByNameAttribute.cs

@@ -2,8 +2,12 @@
 
 namespace Avalonia.Controls
 {
+    /// <summary>
+    /// Indicates that the property resolves an element by Name or x:Name.
+    /// When applying this to attached properties, ensure to put on both
+    /// the Getter and Setter methods.
+    /// </summary>
     public class ResolveByNameAttribute : Attribute
     {
-
     }
 }