What does this implement/fix?

No functional change This PR addresses a message which unnecessarily concerned a user in the related Discourse thread. We solve this by not logging this message unless debug.config = true is explicitly set. There is no issue at all when this message appears. It simply means the config wasn't old - as the old item wasn't there, it didn't need migration.

While I was at it, I expanded the comments around this code to make clear what the code does and why it may reach one or the other place.

Related issue or feature (if applicable): see link above

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

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I have squashed any insignificant commits. (git rebase)

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[x] The code change is tested and works locally.
[x] I based my code and PRs against the repositories developmental branch.
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pi-hole / FTL

Improve config migration logging #1891

What does this implement/fix?

Checklist: