docs(logging): add placeholders for dynamic data

Author: alanpoulain

logging.rst

@@ -29,6 +29,11 @@ To log a message, inject the default logger in your controller or service::
 
     public function index(LoggerInterface $logger)
     {
+        // always use placeholders for dynamic data
+        $logger->debug('User {userId} has logged in', [
+            'userId' => $this->getUserId(),
+        ]);
+
         $logger->info('I just got the logger');
         $logger->error('An error occurred');