Docs feedback: Add info that domain can only be verified for one group

In the following docs, we talk about domain verification:

• https://docs.gitlab.com/ee/user/project/pages/custom_domains_ssl_tls_certification/index.html#prerequisites
• https://docs.gitlab.com/ee/user/enterprise_user/#set-up-a-verified-domain

However, we don't mention that each domain (or sub-domain) can only be verified for one group. I think that most readers probably intuit this, but I think we should still state it.

A customer opened ticket #425784 (GitLab Internal), where they stated they have 2 top-level groups using the same domain for their SCIM configuration. In this scenario, only one of the top-level groups will be able to verify the domain, meaning that the other group will not be able to take advantage of features relating to domain verification (like enterprise users).

At a minimum, I think we should add a message somewhere in both docs that states that a domain can only be verified for one group at a time. I don't know the best way to phrase this, or how to display it (standalone warning? prerequisite? etc.?), so I'm opening this issue rather than an MR.

I'm going to label this as groupknowledge since that's who owns the GitLab Pages doc, and that one informs the enterprise users doc. I think that ~"group::authentication and authorization" is also relevant here, though.