Add doc links to RawGestureDetector (#81796)

packages/flutter/lib/src/gestures/recognizer.dart

@@ -51,7 +51,8 @@ enum DragStartBehavior {
 ///
 /// See also:
 ///
-///  * [GestureDetector], the widget that is used to detect gestures.
+///  * [GestureDetector], the widget that is used to detect built-in gestures.
+///  * [RawGestureDetector], the widget that is used to detect custom gestures.
 ///  * [debugPrintRecognizerCallbacksTrace], a flag that can be set to help
 ///    debug issues with gesture recognizers.
 abstract class GestureRecognizer extends GestureArenaMember with DiagnosticableTreeMixin {

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

@@ -201,6 +201,7 @@ class GestureRecognizerFactoryWithHandlers<T extends GestureRecognizer> extends
 ///  * [Listener], a widget for listening to lower-level raw pointer events.
 ///  * [MouseRegion], a widget that tracks the movement of mice, even when no
 ///    button is pressed.
+///  * [RawGestureDetector], a widget that is used to detect custom gestures.
 class GestureDetector extends StatelessWidget {
   /// Creates a widget that detects gestures.
   ///