Clarify the role of dns.domain outside of the DHCP context

[DL6ER]

What does this implement/fix?

See title and #2235 for additional context.

Related issue or feature (if applicable): Fixes #2235

Pull request in docs with documentation (if applicable): N/A

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[mwoolweaver]

The DNS domain used by your Pi-hole.\n\n This DNS domain in purely local.

Should this say "domain is purely local"?

[DL6ER]

I'm going to assume the approvals are still valid after fixing only a one-letter typo

[yubiuser]

web port PR pi-hole/web#3245