Extend documentation about db:check-migrations

What does this MR do and why?

I feel like it's not clear in our docs what the db:check-migration does, why it does, and where it does.

I've also seen some MRs where the job fails the schema comparison just because the MR is behind the target branch.

So, my proposal is to extend this documentation to hopefully bring clarity to developers as well as provide a doc reference for reviewers when explaining what's behind this job.

This is a follow-up from an MR where this happened (!78560 (comment 815328950)):

I also noticed that the db:check-migrations job failed, but it looks unrelated. Is this normal or do I need to do something? 🤔

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