doc: Update information about supported Git auth backends
    
    We now have a new pagure_authorized_keys backend and we want
    to make clear what the difference is between the pagure backend
    and the pagure_authorized_keys backend and how to set them up.
    
    Also, make clear that an additional setting is required for using
    Gitolite backends.
    
    Signed-off-by: Neal Gompa <ngompa13@gmail.com>