Add documentation for pending vulnerability_occurrences migration

What does this MR do and why?

Addresses #356871 (closed)

!72788 (merged) which was added in %14.5 did not call Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded in the background migration which causes the finished jobs to be in pending state. This MR updates the documentation to mark all the jobs as succeeded.

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.