minor #17977 Add placeholders for dynamic data (alanpoulain)

This PR was merged into the 5.4 branch. Discussion ---------- Add placeholders for dynamic data From `@Crell`'s post: > Symfony's docs team can include a better example that includes a placeholder on the logging page. It doesn't have to be long, just one more example at the top showing how the placeholder works. So here it is 😄 Commits ------- fcc67c9 docs(logging): add placeholders for dynamic data

Author: javiereguiluz

logging.rst

@@ -32,6 +32,15 @@ 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 (wrap the placeholder name with braces)
+ // these placeholders are useful for logging tools, which can aggregate log messages
+ // that are the same except for some variable values inside them, for translation
+ // systems in order to create localized messages, and for security because escaping
+ // can then be done by the implementation in a context-aware fashion
+ $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',