Document the new behavior

src/Avalonia.ReactiveUI/ReactiveUserControl.cs

@@ -7,8 +7,9 @@ using ReactiveUI;
 namespace Avalonia.ReactiveUI
 {
     /// <summary>
-    /// A ReactiveUI UserControl that implements <see cref="IViewFor{TViewModel}"/> 
-    /// and will activate your ViewModel automatically if it supports activation.
+    /// A ReactiveUI UserControl that implements <see cref="IViewFor{TViewModel}"/> and will activate your ViewModel
+    /// automatically if it supports activation. When the DataContext property changes, this class will update the
+    /// ViewModel property with the new DataContext value, and vice versa.
     /// </summary>
     /// <typeparam name="TViewModel">ViewModel type.</typeparam>
     public class ReactiveUserControl<TViewModel> : UserControl, IViewFor<TViewModel> where TViewModel : class

src/Avalonia.ReactiveUI/ReactiveWindow.cs

@@ -7,8 +7,9 @@ using ReactiveUI;
 namespace Avalonia.ReactiveUI
 {
     /// <summary>
-    /// A ReactiveUI Window that implements <see cref="IViewFor{TViewModel}"/>
-    /// and will activate your ViewModel automatically if it supports activation.
+    /// A ReactiveUI Window that implements <see cref="IViewFor{TViewModel}"/> and will activate your ViewModel
+    /// automatically if it supports activation. When the DataContext property changes, this class will update the
+    /// ViewModel property with the new DataContext value, and vice versa.
     /// </summary>
     /// <typeparam name="TViewModel">ViewModel type.</typeparam>
     public class ReactiveWindow<TViewModel> : Window, IViewFor<TViewModel> where TViewModel : class