Link the API docs on waitFor() to the docs for runUnsynchronized() (#69761)

packages/flutter_driver/lib/src/driver/driver.dart

@@ -205,6 +205,14 @@ abstract class FlutterDriver {
   }
 
   /// Waits until [finder] locates the target.
+  ///
+  /// The [finder] will wait until there is no pending frame scheduled
+  /// in the app under test before executing an action.
+  ///
+  /// See also:
+  ///
+  ///  * [FlutterDriver.runUnsynchronized], which will execute an action
+  ///    with frame sync disabled even while frames are pending.
   Future<void> waitFor(SerializableFinder finder, { Duration timeout }) async {
     await sendCommand(WaitFor(finder, timeout: timeout));
   }