Clarify RenderObjectWidget.updateRenderObject (#6702)

Closes https://github.com/flutter/flutter/issues/5743

packages/flutter/lib/src/widgets/framework.dart

@@ -1166,12 +1166,24 @@ abstract class RenderObjectWidget extends Widget {
   /// Creates an instance of the [RenderObject] class that this
   /// [RenderObjectWidget] represents, using the configuration described by this
   /// [RenderObjectWidget].
+  ///
+  /// This method should not do anything with the children of the render object.
+  /// That should instead be handled by the method that overrides
+  /// [RenderObjectElement.mount] in the object rendered by this object's
+  /// [createElement] method. See, for example,
+  /// [SingleChildRenderObjectElement.mount].
   @protected
   RenderObject createRenderObject(BuildContext context);
 
   /// Copies the configuration described by this [RenderObjectWidget] to the
   /// given [RenderObject], which will be of the same type as returned by this
   /// object's [createRenderObject].
+  ///
+  /// This method should not do anything to update the children of the render
+  /// object. That should instead be handled by the method that overrides
+  /// [RenderObjectElement.update] in the object rendered by this object's
+  /// [createElement] method. See, for example,
+  /// [SingleChildRenderObjectElement.update].
   @protected
   void updateRenderObject(BuildContext context, @checked RenderObject renderObject) { }