Mention job tracking in background migration cleaning up doc

What does this MR do and why?

Related to #346606 (closed)

In !73847 (comment 725631702) we discussed how some background migration helpers relay on job tracking. However, this is not mentioned in the background migration docs. For example, the "Cleaning up" section suggests using finalize_background_migration but this only works when the previous migration used the queue_background_migration_jobs_by_range_at_intervals helper.

This MR adds notes about job tracking to the cleaning up section

Screenshots or screen recordings

How to set up and validate locally

1. Visit http://localhost:3000/help/development/background_migrations.html#cleaning-up

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