Add comment to background migration finalize

What does this MR do and why?

Adds a comment to the finalize_background_migration helper describing that it can only be used of background migration jobs had been created.

In !73495 (comment 723686564) I was wondering if this helper could be used but then discovered that it would only work if the previous migration produced tracking jobs

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.