Add documentation for how to use annotated region - #24989

  /// {@tool snippet --template=stateful_widget}
  /// The following example creates a widget that changes the status bar color
  /// to a random value on Android.
  ///
  /// ```dart
  ///    final _random = math.Random();
  ///    SystemUiOverlayStyle _currentStyle = SystemUiOverlayStyle.light;
  ///
  ///    void _changeColor() {
  ///      final color = Color.fromRGBO(
  ///        _random.nextInt(255),
  ///        _random.nextInt(255),
  ///        _random.nextInt(255),
  ///        1.0,
  ///      );
  ///      setState(() {
  ///        _currentStyle = SystemUiOverlayStyle.dark.copyWith(
  ///          statusBarColor: color,
  ///        );
  ///      });
  ///    }
  ///
  ///    @override
  ///    Widget build(BuildContext context) {
  ///      return Scaffold(
  ///        body: AnnotatedRegion(
  ///          sized: false,
  ///          value: _currentStyle,
  ///           child: Center(
  ///             child: RaisedButton(
  ///               child: const Text('Change Color'),
  ///               onPressed: _changeColor,
  ///             ),
  ///           ),
  ///         ),
  ///       );
  ///     }
  /// {@end-tool}