Add help string on how to configure multiple DNS forwards for various cli tools
    
    The man pages for various FreeIPA setup tools are more descriptive on how to
    configure multiple DNS forwarders than the corresponding cli help. This patch
    makes the cli help more verbose now for the following tools:
    
    * ipa-dns-install
    * ipa-replica-install
    * ipa-server-install
    
    https://fedorahosted.org/freeipa/ticket/4465
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>