Link to FadeTransition from performance tip (#86379)

When suggesting more performant alternatives to rebuilding `Opacity`, `FadeTransition` is as valid as `AnimatedOpacity`, and it's less obvious ("opacity" versus "fade").

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

@@ -197,7 +197,7 @@ class Directionality extends InheritedWidget {
 ///
 /// Animating an [Opacity] widget directly causes the widget (and possibly its
 /// subtree) to rebuild each frame, which is not very efficient. Consider using
-/// an [AnimatedOpacity] instead.
+/// an [AnimatedOpacity] or a [FadeTransition] instead.
 ///
 /// ## Transparent image
 ///