Update src/Avalonia.Controls/Notifications/NotificationPosition.cs

improve xml comments

Co-Authored-By: Steven Kirk <grokys@users.noreply.github.com>

src/Avalonia.Controls/Notifications/NotificationPosition.cs

@@ -4,7 +4,8 @@
 namespace Avalonia.Controls.Notifications
 {
     /// <summary>
-    /// Enumeration of Notifications positions.
+    /// Describes the possible positions for <see cref="Notification"/>s that are displayed by a
+    /// <see cref="WindowNotificationManager"/>.
     /// </summary>
     public enum NotificationPosition
     {