You could adapt what’s documented there if –IF– you need DynDNS. I’d suggest to assign fixed IP addresses in the DNS application of UCS instead.
Generally, I’d advise against editing the config files directly. They are generated from templates in
/etc/univention/templates/files. For almost all services UCS provides there are template files it generates the configs from. Each config file that’s generated in such a way has a header referencing the templates it was built upon.
It’s still not ideal to edit templates (see above) but better than editing the config files themselves. I’m not sure what Univention’s stance on editing template files is.
To return to your issue: You have two tasks at hand. You want your DNS server to know about the IP addresses of your clients. As I don’t use the DHCP server in UCS I don’t know if it tells bind9 about the dynamic clients. If your clients are members of the domain you could just assign fixed IP addresses. Well, you could assign fixed IP addresses even if they are not members of your domain.
If you don’t want to assign fixed IP address ISC-DHCP has to update the zone info of bind9. I did this once manually and I really hope that Univention thought of linking those two services as it’s no trivial task as you can see by reading the instructions you linked. Either way, this is how your DNS will know about the clients.
For your clients to reolve hostnames by implicitly adding your domain name you need to tell them the search-domain.