add UserControl comment

src/Avalonia.Controls/UserControl.cs

@@ -6,6 +6,9 @@ using Avalonia.Styling;
 
 namespace Avalonia.Controls
 {
+    /// <summary>
+    /// Provides the base class for defining a new control that encapsulates related existing controls and provides its own logic.
+    /// </summary>
     public class UserControl : ContentControl, IStyleable, INameScope
     {
         private readonly NameScope _nameScope = new NameScope();
@@ -24,6 +27,7 @@ namespace Avalonia.Controls
             remove { _nameScope.Unregistered -= value; }
         }
 
+        /// <inheritdoc/>
         Type IStyleable.StyleKey => typeof(ContentControl);
 
         /// <inheritdoc/>