Clarifying some info about sidekiq jobs in background migrations

doc/development/background_migrations.md

-# Background Migrations
+---
+type: reference, dev
+stage: none
+group: Development
+info: "See the Technical Writers assigned to Development Guidelines: https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments-to-development-guidelines"
+---
+
+# Background migrations
 
 Background migrations can be used to perform data migrations that would
 otherwise take a very long time (hours, days, years, etc) to complete. For
@@ -92,6 +99,20 @@ bulk_migrate_async(
 )
 ```
 
+Note that this will queue a Sidekiq job immediately: if you have a large number
+of records, this may not be what you want. You can use the function
+`queue_background_migration_jobs_by_range_at_intervals` to split the job into
+batches:
+
+```ruby
+queue_background_migration_jobs_by_range_at_intervals(
+  ClassName,
+  BackgroundMigrationClassName,
+  2.minutes,
+  batch_size: 10_000
+  )
+```
+
 You'll also need to make sure that newly created data is either migrated, or
 saved in both the old and new version upon creation. For complex and time
 consuming migrations it's best to schedule a background job using an