Update shibboleth auth documentation

[duritong]

Since introducing jicofo.conf it seems that the documentation how to add shibboleth authentication is not anymore up to date: https://github.com/jitsi/jicofo/blob/master/doc/shibboleth.md

The documentation should be updated to clarify how shibboleth auth should work with the new jicofo.conf configuration

See related the question how to configure shibboleth auth in docker environments jitsi/docker-jitsi-meet#956