Add CupertinoScrollbar api docs (#78284)

packages/flutter/lib/src/cupertino/scrollbar.dart

@@ -38,7 +38,72 @@ const double _kScrollbarCrossAxisMargin = 3.0;
 /// animate from [thickness] and [radius] to [thicknessWhileDragging] and
 /// [radiusWhileDragging], respectively.
 ///
-// TODO(Piinks): Add code sample
+/// {@tool dartpad --template=stateless_widget_scaffold}
+/// This sample shows a [CupertinoScrollbar] that fades in and out of view as scrolling occurs.
+/// The scrollbar will fade into view as the user scrolls, and fade out when scrolling stops.
+/// The `thickness` of the scrollbar will animate from 6 pixels to the `thicknessWhileDragging` of 10
+/// when it is dragged by the user. The `radius` of the scrollbar thumb corners will animate from 34
+/// to the `radiusWhileDragging` of 0 when the scrollbar is being dragged by the user.
+///
+/// ```dart imports
+/// import 'package:flutter/cupertino.dart';
+/// ```
+///
+/// ```dart
+/// @override
+/// Widget build(BuildContext context) {
+///   return CupertinoScrollbar(
+///     thickness: 6.0,
+///     thicknessWhileDragging: 10.0,
+///     radius: const Radius.circular(34.0),
+///     radiusWhileDragging: Radius.zero,
+///     child: ListView.builder(
+///       itemCount: 120,
+///       itemBuilder: (BuildContext context, int index) {
+///         return Center(
+///           child: Text('item $index'),
+///         );
+///       },
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
+///
+/// {@tool dartpad --template=stateful_widget_scaffold}
+/// When `isAlwaysShown` is true, the scrollbar thumb will remain visible without the
+/// fade animation. This requires that a [ScrollController] is provided to controller,
+/// or that the [PrimaryScrollController] is available.
+///
+/// ```dart imports
+/// import 'package:flutter/cupertino.dart';
+/// ```
+///
+/// ```dart
+/// final ScrollController _controllerOne = ScrollController();
+///
+/// @override
+/// Widget build(BuildContext context) {
+///   return CupertinoScrollbar(
+///     thickness: 6.0,
+///     thicknessWhileDragging: 10.0,
+///     radius: const Radius.circular(34.0),
+///     radiusWhileDragging: Radius.zero,
+///     controller: _controllerOne,
+///     isAlwaysShown: true,
+///     child: ListView.builder(
+///       controller: _controllerOne,
+///       itemCount: 120,
+///       itemBuilder: (BuildContext context, int index) {
+///         return Center(
+///           child: Text('item $index'),
+///         );
+///       },
+///     ),
+///   );
+/// }
+/// ```
+/// {@end-tool}
 ///
 /// See also:
 ///