[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 | 
> A) Do you think we could/should add a message to the ``server:start`` command?

This sounds reasonable to me. Especially, since we just added a similar message to the ``server:run`` command. By the way, that message should probably be ported back to 2.3 and 2.5. What do you think?

> Right now, if I already have a server running on, for example, port 8000, when I run ``server:start``, it just gives me the normal "success" message: ``Web server listening on http://127.0.0.1:8000``. While that's technically true, the reality is that the server did not start, because the port was already taken. Can we handle this better?

Actually, I thought that it wouldn't print a success message. To me, this is not what I would have expected in this case and I would consider it to be a bug. Do you like to open an issue? I'll have a look if there is a way to handle this somehow in a nice way.