MAN: hint nested groups by simple access provider
    
    sssd-ldap hints to use the simple access provider if a nested group membership
    is needed. Add explicit notice in sssd-simple about support of nested group
    membership.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2308
    
    Reviewed-by: Stephen Gallagher <sgallagh@redhat.com>