Sub-domain delegation Use Case should specific the Server

Issue in ./help/subdomains-certificates/using/setting-up-new-subdomain.md

When setting up a sub-domain it is very important that the sub-domain is delegated to the correct server (MKT or RT) during the delegation. The Use Case definition does not indicate which Server it is tied to and that can be misleading as customer can send Transactional Type emails from the MKT Server and they can send Marketing type emails from the RT Server. When they choose the incorrect Use Case (server) that leads to an incorrect setup of the sub-domains throughout the platform and then the sub-domains have to be un-delegated and the process restarted.

I believe the Use Case section should really be titled to indicate which server (MKT or RT) they are looking to apply the sub-domain for as that is what we need to know for proper setup on the correct instance.

Thanks,
Michael

Open help/subdomains-certificates/using/setting-up-new-subdomain.md and locate the Use Case section. Update its title and wording so the applicable server, MKT or RT, is explicit for sub-domain delegation. Done means customers can identify the correct server before starting setup.

Written by the indexing model from the issue text.