Add documentation for drag/fling offset in WidgetController. (#118288)

* Documentation for drag/fling offset

* Fix typo

* Fix typo 2

* Fix the docs_test

* Fix the grammar

Author: peixinli

packages/flutter_test/lib/src/controller.dart

@@ -562,6 +562,16 @@ abstract class WidgetController {
   ///
   /// {@macro flutter.flutter_test.WidgetController.tap.warnIfMissed}
   ///
+  /// {@template flutter.flutter_test.WidgetController.fling.offset}
+  /// The `offset` represents a distance the pointer moves in the global
+  /// coordinate system of the screen.
+  ///
+  /// Positive [Offset.dy] values mean the pointer moves downward. Negative
+  /// [Offset.dy] values mean the pointer moves upwards. Accordingly, positive
+  /// [Offset.dx] values mean the pointer moves towards the right. Negative
+  /// [Offset.dx] values mean the pointer moves towards left.
+  /// {@endtemplate}
+  ///
   /// {@template flutter.flutter_test.WidgetController.fling}
   /// This can pump frames.
   ///
@@ -817,6 +827,8 @@ abstract class WidgetController {
   /// The operation happens at once. If you want the drag to last for a period
   /// of time, consider using [timedDrag].
   ///
+  /// {@macro flutter.flutter_test.WidgetController.fling.offset}
+  ///
   /// {@template flutter.flutter_test.WidgetController.drag}
   /// By default, if the x or y component of offset is greater than
   /// [kDragSlopDefault], the gesture is broken up into two separate moves
@@ -953,6 +965,8 @@ abstract class WidgetController {
   ///
   /// {@macro flutter.flutter_test.WidgetController.tap.warnIfMissed}
   ///
+  /// {@macro flutter.flutter_test.WidgetController.fling.offset}
+  ///
   /// This is the timed version of [drag]. This may or may not result in a
   /// [fling] or ballistic animation, depending on the speed from
   /// `offset/duration`.