Add sample code of GestureDetector with no children (#57838)

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

@@ -51,6 +51,7 @@ export 'package:flutter/rendering.dart' show RenderSemanticsGestureHandler;
 // bool _lights;
 // void setState(VoidCallback fn) { }
 // String _last;
+// Color _color;
 
 /// Factory for creating gesture recognizers.
 ///
@@ -130,8 +131,9 @@ class GestureRecognizerFactoryWithHandlers<T extends GestureRecognizer> extends
 ///
 /// {@tool snippet}
 ///
-/// This example turns the light bulb yellow when the "turn lights on" button is
-/// tapped by setting the `_lights` field:
+/// This example of a [Container] contains a black light bulb wrapped in a [GestureDetector].
+/// It turns the light bulb yellow when the "turn lights on" button is tapped
+/// by setting the `_lights` field. Above animation shows the code in use:
 ///
 /// ```dart
 /// Container(
@@ -166,6 +168,29 @@ class GestureRecognizerFactoryWithHandlers<T extends GestureRecognizer> extends
 /// ```
 /// {@end-tool}
 ///
+/// {@tool snippet}
+///
+/// This example of a [Container] wraps a [GestureDetector] widget.
+/// Since the [GestureDetector] does not have a child it takes on the size of
+/// its parent making the entire area of the surrounding [Container] clickable.
+/// When tapped the [Container] turns yellow by setting the `_color` field:
+///
+/// ```dart
+/// Container(
+///   color: _color,
+///   height: 200.0,
+///   width: 200.0,
+///   child: GestureDetector(
+///     onTap: () {
+///       setState(() {
+///         _color = Colors.yellow;
+///       });
+///     },
+///   ),
+/// )
+/// ```
+/// {@end-tool}
+///
 /// ## Debugging
 ///
 /// To see how large the hit test box of a [GestureDetector] is for debugging