Emphasize the behavior property in GestureDetector's class doc. (#12536)

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

@@ -95,6 +95,9 @@ class GestureRecognizerFactoryWithHandlers<T extends GestureRecognizer> extends
 /// If this widget has a child, it defers to that child for its sizing behavior.
 /// If it does not have a child, it grows to fit the parent instead.
 ///
+/// By default a GestureDetector with an invisible child ignores touches;
+/// this behavior can be controlled with [behavior].
+///
 /// GestureDetector also listens for accessibility events and maps
 /// them to the callbacks. To ignore accessibility events, set
 /// [excludeFromSemantics] to true.