Update BBM docs to include generator details
What does this MR do and why?
Updates https://docs.gitlab.com/ee/development/database/batched_background_migrations.html to include BBM generator introduced in !111102 (merged)
MR acceptance checklist
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I have evaluated the MR acceptance checklist for this MR.
Closes #388301 (closed)
Edited by Prabakaran Murugesan