Add info about security.firewall.context in pitfall

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 
I spent some time for looking into the source code to understand, what does the security/firewalls/{id}/context parameter do. The user guide for SonataUserBundle has an example with this parameter, but I did not found any information. The links in search results are all about the service "security.context".