Improve docs of ImageFiltered and BackdropFilter (#65503)

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

@@ -384,8 +384,14 @@ class ShaderMask extends SingleChildRenderObjectWidget {
 /// This effect is relatively expensive, especially if the filter is non-local,
 /// such as a blur.
 ///
+/// If all you want to do is apply an [ImageFilter] to a single widget
+/// (as opposed to applying the filter to everything _beneath_ a widget), use
+/// [ImageFiltered] instead. For that scenario, [ImageFiltered] is both
+/// easier to use and less expensive than [BackdropFilter].
+///
 /// See also:
 ///
+///  * [ImageFiltered], which applies an [ImageFilter] to its child.
 ///  * [DecoratedBox], which draws a background under (or over) a widget.
 ///  * [Opacity], which changes the opacity of the widget itself.
 class BackdropFilter extends SingleChildRenderObjectWidget {

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

@@ -12,6 +12,12 @@ import 'package:flutter/rendering.dart';
 import 'framework.dart';
 
 /// Applies an [ImageFilter] to its child.
+///
+/// See also:
+///
+/// * [BackdropFilter], which applies an [ImageFilter] to everything
+///   beneath its child.
+/// * [ColorFiltered], which applies a [ColorFilter] to its child.
 @immutable
 class ImageFiltered extends SingleChildRenderObjectWidget {
   /// Creates a widget that applies an [ImageFilter] to its child.