Cookbook for session concurrency control

This PR documents the symfony/symfony#12009.

Any feedback is welcome as my English is really bad :(
What about rewording the introductory paragraph as follows?

```
Sometimes, it's useful to restrict the number of concurrent sessions that a given
user can open using different browsers and devices. When the maximum number of
concurrent sessions is achieved, Symfony allows you to disable new sessions or to
expire the old ones. To do so, set the ``max_sessions`` limit using the ``session_concurrency``
option of your firewall:
```