BBM: How-it-works section update
What does this MR do and why?
Follow-up from !122513 (merged)
Updates the `Execution mechanism` section to:
After being enqueued, batched background migrations are picked in parallel order by a Sidekiq worker. At this point, batched jobs are created - or retried and iterate over a range of records that are fetched based on the configuration defined inside the migration class. Each job instance receives a set of arguments based on this migration configuration. After each job execution, the optimization mechanism takes place to decide if the migration can be optimized.
GitLab.com is configured to run 4 parallel BBMs, while other instances uses a default value of 2 parallel migrations.
Soon as a worker is available, the BBM is processed by the runner.
MR acceptance checklist
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Related to #388789 (closed)