improve "no device connected" message after "flutter run" (#6179)

packages/flutter_tools/lib/src/commands/run.dart

@@ -114,6 +114,8 @@ class RunCommand extends RunCommandBase {
       printStatus('To run on a simulator, launch it first:');
       printStatus('open -a Simulator.app');
       printStatus('');
+      printStatus('If you expected your device to be detected, please run "flutter doctor" to diagnose');
+      printStatus('potential issues, or visit https://flutter.io/setup/ for troubleshooting tips.');
     }
   }