[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 | 
Hey!

1) I see you backported the message about server:run to 2.3 just now. Awesome.

2) I opened an issue! symfony/symfony#12259

3) Yes, let's remove the intro and the `server:stop` part. Or really, the intro should just be slightly different. Since we're talking about permissions here, something like this might work better:

>
The built-in PHP web server - which can be used during development - allows your
web server user and CLI user to be the same. This removes any permissions issues.