Docs: note that domain restrictions only apply to groups, not projects

What does this MR do?

Adds a note to the Domain Restriction section to ensure it is clear that the group domain restrictions do not apply to projects owned by those groups and that users may still be added with email addresses whose domains do not fall under the restriction. This was a concern raised by a large Silver customer in this support case (GitLab internal link) and a feature request to add the restriction to projects exists here

Related issues

#34403

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When applicable:

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