[Cookbook][Web server] description for running PHP's built-in web server in the background

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11311)
| Applies to    | 2.6+
| Fixed tickets | 
> Hmm, I think we should maybe keep the old section with some changes, because using the built-in PHP web server is not the only way to run things with the same user.

Hm, that's maybe true. We should then just make it not as important as the built-in server. What do you think about keeping it as item 5 below the web server?