Add info that domain can only be verified for one group

What does this MR do?

Clarifies that domains can only be verified for one group at a time.

For example, we have two top-level groups, A and B, and they each have multiple nested sub-groups and projects. We also have two domains for verification, X and Y.

And, as the enterprise user docs state, domain verification applies at the top-level group and to all subgroups and projects nested under that top-level parent group.

So:

• Domain X can be verified for group A and its nested sub-groups and projects, domain Y can be verified for group B and its nested sub-groups and projects, or vice versa.
• However, domain X cannot be verified for both group A and group B, and the same applies to domain Y. It's a 1-2-1 exclusive relationship between the domain and the top-level group.
• The presence of nested sub-groups and projects does not change that.

Related issues

#417050 (closed)

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