Update Draggable API Docs (#69527)

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

@@ -99,6 +99,73 @@ enum DragAnchor {
 ///
 /// {@youtube 560 315 https://www.youtube.com/watch?v=QzA4c4QHZCY}
 ///
+/// {@tool dartpad --template=stateful_widget_material}
+///
+/// The following example has a [Draggable] widget along with a [DragTarget]
+/// in a row demonstrating an incremented `acceptedData` integer value when
+/// you drag the element to the target.
+///
+/// ```dart
+/// int acceptedData = 0;
+/// Widget build(BuildContext context) {
+///   return Row(
+///     mainAxisAlignment: MainAxisAlignment.spaceEvenly,
+///     children: [
+///       Draggable<int>(
+///         // Data is the value this Draggable stores.
+///         data: 10,
+///         child: Container(
+///           height: 100.0,
+///           width: 100.0,
+///           color: Colors.lightGreenAccent,
+///           child: Center(
+///             child: Text("Draggable"),
+///           ),
+///         ),
+///         feedback: Container(
+///           color: Colors.deepOrange,
+///           height: 100,
+///           width: 100,
+///           child: Icon(Icons.directions_run),
+///         ),
+///         childWhenDragging: Container(
+///           height: 100.0,
+///           width: 100.0,
+///           color: Colors.pinkAccent,
+///           child: Center(
+///             child: Text("Child When Dragging"),
+///           ),
+///         ),
+///       ),
+///       DragTarget(
+///         builder: (
+///           BuildContext context,
+///           List<dynamic> accepted,
+///           List<dynamic> rejected,
+///         ) {
+///           return Container(
+///             height: 100.0,
+///             width: 100.0,
+///             color: Colors.cyan,
+///             child: Center(
+///               child: Text("Value is updated to: $acceptedData"),
+///             ),
+///           );
+///         },
+///         onAccept: (int data) {
+///           setState(() {
+///             acceptedData += data;
+///           });
+///         },
+///       ),
+///     ],
+///   );
+/// }
+///
+/// ```
+///
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [DragTarget]