[php-src] Issue #14474: Better document remote user in PHP-FPM access.format

Issue: https://github.com/php/php-src/issues/14474 Author: RoSk0 ### Description Default [PHP-FPM pool configuration file](https://github.com/php/php-src/blob/master/sapi/fpm/www.conf.in#L351) has the %u option described as remote user in the access.format documentation, but what is a remote user? In my set up I have PHP-FPM and Nginx talking on the port 9000 on localhost (Kubernetes pod) and user field is always empty in the logs. I don't expect PHP-FPM to know about my application users, but what should I expect in that field?