[Security] Missing documentation regarding AccessDeniedHandlerInterface

[linaori]

I've seen a few occasion where custom access denied handling is required. It's not rocket science to implement, you put access_denied_handler: service.id on your firewall configuration and implement the AccessDeniedHandlerInterface in this service.

I think this could be a simple explanation but add some real value to the docs.

[wouterj]

I vote to add this information to reference/configuration/framework