Update dragDevices doc to include default PointerDeviceKind.trackpad (#163898)

[ScrollBehavior.dragDevices](https://api.flutter.dev/flutter/widgets/ScrollBehavior/dragDevices.html)
docs say

> By default only PointerDeviceKind.touch, PointerDeviceKind.stylus, and
PointerDeviceKind.invertedStylus are configured to create drag gestures.

However, `PointerDeviceKind.trackpad` is also included by default
https://github.com/flutter/flutter/pull/89944/


401ab831ee/packages/flutter/lib/src/widgets/scroll_configuration.dart (L115-L119)


401ab831ee/packages/flutter/lib/src/widgets/scroll_configuration.dart (L33)

Update the comment to include `trackpad`:
> By default only PointerDeviceKind.touch, PointerDeviceKind.stylus,
PointerDeviceKind.invertedStylus, **and PointerDeviceKind.trackpad** are
configured to create drag gestures.



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

@@ -112,10 +112,11 @@ class ScrollBehavior {
 
   /// The device kinds that the scrollable will accept drag gestures from.
   ///
-  /// By default only [PointerDeviceKind.touch], [PointerDeviceKind.stylus], and
-  /// [PointerDeviceKind.invertedStylus] are configured to create drag gestures.
-  /// Enabling this for [PointerDeviceKind.mouse] will make it difficult or
-  /// impossible to select text in scrollable containers and is not recommended.
+  /// By default only [PointerDeviceKind.touch], [PointerDeviceKind.stylus],
+  /// [PointerDeviceKind.invertedStylus], and [PointerDeviceKind.trackpad]
+  /// are configured to create drag gestures. Enabling this for
+  /// [PointerDeviceKind.mouse] will make it difficult or impossible to select
+  /// text in scrollable containers and is not recommended.
   Set<PointerDeviceKind> get dragDevices => _kTouchLikeDeviceTypes;
 
   /// {@macro flutter.gestures.monodrag.DragGestureRecognizer.multitouchDragStrategy}