Update comment as well.

src/Avalonia.Controls/ItemsControl.cs

@@ -237,14 +237,8 @@ namespace Avalonia.Controls
         /// Creates the <see cref="ItemContainerGenerator"/> for the control.
         /// </summary>
         /// <returns>
-        /// An <see cref="IItemContainerGenerator"/> or null.
+        /// An <see cref="IItemContainerGenerator"/>.
         /// </returns>
-        /// <remarks>
-        /// Certain controls such as <see cref="TabControl"/> don't actually create item 
-        /// containers; however they want it to be ItemsControls so that they have an Items 
-        /// property etc. In this case, a derived class can override this method to return null
-        /// in order to disable the creation of item containers.
-        /// </remarks>
         protected virtual IItemContainerGenerator CreateItemContainerGenerator()
         {
             return new ItemContainerGenerator(this);