Merge branch '5.4' into 6.2

* 5.4:
  docs(logging): add placeholders for dynamic data

Author: javiereguiluz

logging.rst

@@ -32,6 +32,12 @@ To log a message, inject the default logger in your controller or service::
         $logger->info('I just got the logger');
         $logger->error('An error occurred');
 
+        // log messages can also contain placeholders, which are variable names
+        // wrapped in braces whose values are passed as the second argument
+        $logger->debug('User {userId} has logged in', [
+            'userId' => $this->getUserId(),
+        ]);
+
         $logger->critical('I left the oven on!', [
             // include extra "context" info in your logs
             'cause' => 'in_hurry',
@@ -40,6 +46,14 @@ To log a message, inject the default logger in your controller or service::
         // ...
     }
 
+Adding placeholders to log messages is recommended because:
+
+* It's easier to check log messages because many logging tools group log messages
+  that are the same except for some variable values inside them;
+* It's much easier to translate those log messages;
+* It's better for security, because escaping can then be done by the
+  implementation in a context-aware fashion.
+
 The ``logger`` service has different methods for different logging levels/priorities.
 See `LoggerInterface`_ for a list of all of the methods on the logger.