Clarify BG migration docs about the finalization policy and retry workflow

What does this MR do?

This MR clarifies the expectation of the process flow on BG migrations. Currently, it's unclear that the answer could vary per DB maintainer, for example,

• Where is the border to allow finalization? For example, do we allow 90 minutes finalization?
• If a migration failure happens and the root cause is NOT originated by the migration itself (example), what developers should do?

See more discussions in !68763 (comment 660941824)

Related to

• Fix BG migration causes many unprocessed jobs due to the retry strategy with concurrency limit
• Re Reschedule Delete Orphaned Deployments BG migration
• Remove inappropriate batch size in requeue background migration jobs
• Finalize delete orphaned deployments background migration

MR acceptance checklist

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