Add a note on JsonFormat annotation and ORM FormatMapper

[marko-bekhta]

So that there's a slightly better chance that the correct annotation is used and not the com.fasterxml.jackson.annotation.JsonFormat one.

[github-actions]

🙈 The PR is closed and the preview is expired.

[quarkus-bot]

Status for workflow Quarkus Documentation CI

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It should be safe to merge provided you have a look at the other checks in the summary.

[yrodiere]

This begs the question: should we rename this annotation, if its current name is confusing?

But in the meantime, I agree this is better.