Mention setState in onChanged docs. (#6442)

Add some docs to explain how to use setState() wiht Checkbox, Radio,
Slider, and Switch.

Based on experience of usability study participant P1.

packages/flutter/lib/src/material/checkbox.dart

@@ -58,6 +58,21 @@ class Checkbox extends StatefulWidget {
   /// value.
   ///
   /// If null, the checkbox will be displayed as disabled.
+  ///
+  /// The callback provided to onChanged should update the state of the parent
+  /// [StatefulWidget] using the [State.setState] method, so that the parent
+  /// gets rebuilt; for example:
+  ///
+  /// ```dart
+  /// new Checkbox(
+  ///   value: _throwShotAway,
+  ///   onChanged: (bool newValue) {
+  ///     setState(() {
+  ///       _throwShotAway = newValue;
+  ///     });
+  ///   },
+  /// ),
+  /// ```
   final ValueChanged<bool> onChanged;
 
   /// The color to use when this checkbox is checked.

packages/flutter/lib/src/material/radio.dart

@@ -70,6 +70,22 @@ class Radio<T> extends StatefulWidget {
   /// radio button with the new [groupValue].
   ///
   /// If null, the radio button will be displayed as disabled.
+  ///
+  /// The callback provided to onChanged should update the state of the parent
+  /// [StatefulWidget] using the [State.setState] method, so that the parent
+  /// gets rebuilt; for example:
+  ///
+  /// ```dart
+  /// new Radio<SingingCharacter>(
+  ///   value: SingingCharacter.lafayette,
+  ///   groupValue: _character,
+  ///   onChanged: (SingingCharacter newValue) {
+  ///     setState(() {
+  ///       _character = newValue;
+  ///     });
+  ///   },
+  /// ),
+  /// ```
   final ValueChanged<T> onChanged;
 
   /// The color to use when this radio button is selected.

packages/flutter/lib/src/material/slider.dart

@@ -78,6 +78,25 @@ class Slider extends StatefulWidget {
   /// value.
   ///
   /// If null, the slider will be displayed as disabled.
+  ///
+  /// The callback provided to onChanged should update the state of the parent
+  /// [StatefulWidget] using the [State.setState] method, so that the parent
+  /// gets rebuilt; for example:
+  ///
+  /// ```dart
+  /// new Slider(
+  ///   value: _duelCommandment.toDouble(),
+  ///   min: 1.0,
+  ///   max: 10.0,
+  ///   divisions: 10,
+  ///   label: '$_duelCommandment',
+  ///   onChanged: (double newValue) {
+  ///     setState(() {
+  ///       _duelCommandment = newValue.round();
+  ///     });
+  ///   },
+  /// ),
+  /// ```
   final ValueChanged<double> onChanged;
 
   /// The minium value the user can select.

packages/flutter/lib/src/material/switch.dart

@@ -61,6 +61,21 @@ class Switch extends StatefulWidget {
   /// value.
   ///
   /// If null, the switch will be displayed as disabled.
+  ///
+  /// The callback provided to onChanged should update the state of the parent
+  /// [StatefulWidget] using the [State.setState] method, so that the parent
+  /// gets rebuilt; for example:
+  ///
+  /// ```dart
+  /// new Switch(
+  ///   value: _giveVerse,
+  ///   onChanged: (bool newValue) {
+  ///     setState(() {
+  ///       _giveVerse = newValue;
+  ///     });
+  ///   },
+  /// ),
+  /// ```
   final ValueChanged<bool> onChanged;
 
   /// The color to use when this switch is on.