Route can pass template parameter to renderLogin

I know there is already one pull request for this #1053, but it haven't moved forward for 3months.

I am creating new one with correct variable name and documentation update, because there wasn't much activity in the already existing one.

This change allows developers to create more login pages without the need to extend FOSUserBundle or SecurityController. The only thing developers have to do is to add new route and change their security settings accordingly to take the new route into account.