Add a cross-reference to MaterialApp from CheckedModeBanner (#4981)

so that folks have some prayer of figuring out from our docs
how to turn it off. :)

@Hixie

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

@@ -162,6 +162,7 @@ class Banner extends StatelessWidget {
 }
 
 /// Displays a [Banner] saying "SLOW MODE" when running in checked mode.
+/// [MaterialApp] builds one of these by default.
 /// Does nothing in release mode.
 class CheckedModeBanner extends StatelessWidget {
   /// Creates a checked mode banner.