minor #17977 Add placeholders for dynamic data (alanpoulain)

This PR was merged into the 5.4 branch.

Discussion
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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
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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',