[Cookbook][WebServer] mention the force option of the server start command

[xabbuh]

In symfony/symfony#15134, a new --force option was introduced for the server:start command which is useful when PHP's built-in web server terminated unexpected, but the lock file does still exist. In this case, the server:start command would refuse to start a new server process telling you that there already is a running process. The --force could then be used to (try to) start a new process anyway.

This could simply be documented by adding a small tip in the chapter that explains how to control the internal web server (see http://symfony.com/doc/current/cookbook/web_server/built_in.html).

[vincentaubert]

what about this version? @xabbuh

[vincentaubert]

fixed