MAN: Document that each provider type uses its own set of options
    
    https://fedorahosted.org/sssd/ticket/2281
    
    Many users were configuring the combination of
    
    id_provider=ad
    access_provider=ldap
    
    expecting that the access_provider would use the same serves the
    id_provider does. That's not the case, each provider type must be
    configured separately.
    
    Reviewed-by: Stephen Gallagher <sgallagh@redhat.com>