update CupertinoDialogAction isDefaultAction documentation (#34163)

packages/flutter/lib/src/cupertino/dialog.dart

@@ -1055,7 +1055,10 @@ class CupertinoDialogAction extends StatelessWidget {
 
   /// Set to true if button is the default choice in the dialog.
   ///
-  /// Default buttons are bold.
+  /// Default buttons have bold text. Similar to
+  /// [UIAlertController.preferredAction](https://developer.apple.com/documentation/uikit/uialertcontroller/1620102-preferredaction),
+  /// but more than one action can have this attribute set to true in the same
+  /// [CupertinoAlertDialog].
   ///
   /// This parameters defaults to false and cannot be null.
   final bool isDefaultAction;