minor #12257 [FrameworkBundle] improve server:run feedback (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[FrameworkBundle] improve server:run feedback

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

In #12253, improved feedback was added to the `server:run` command
instructing the user how to terminate the built-in web server. This
is hereby backported to the `2.3` branch.

Commits
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264c37a [FrameworkBundle] improve server:run feedback

src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php

@@ -110,6 +110,7 @@ EOF
         $router = realpath($router);
 
         $output->writeln(sprintf("Server running on <info>http://%s</info>\n", $input->getArgument('address')));
+        $output->writeln('Quit the server with CONTROL-C.');
 
         $builder = new ProcessBuilder(array(PHP_BINARY, '-S', $input->getArgument('address'), $router));
         $builder->setWorkingDirectory($documentRoot);