Update comment

Co-authored-by: Steven Kirk <grokys@users.noreply.github.com>

src/Avalonia.Controls/Diagnostics/ToolTipDiagnostics.cs

@@ -7,6 +7,9 @@ namespace Avalonia.Controls.Diagnostics
     /// </summary>
     public static class ToolTipDiagnostics
     {
+        /// <summary>
+        /// Provides access to the internal <see cref="ToolTip.ToolTipProperty"/> for use in DevTools.
+        /// </summary>
         public static AvaloniaProperty<ToolTip?> ToolTipProperty = ToolTip.ToolTipProperty;
     }
 }