Update batched background migrations docs

What does this MR do and why?

Adds information about the column_name requirements for batched background migrations.

The column needs to be one the migration can iterate over. This will prevent constraint errors such as the following from occurring:

PG::CheckViolation: ERROR:  new row for relation "batched_background_migrations" violates check constraint "check_max_value_in_range"
DETAIL:  Failing row contains (106, 2024-07-31 13:58:08.840032+00, 2024-07-31 13:58:08.840032+00, 1, 0, 1000, 100, 120, 1, AlterWebhookDeletedAuditEvent, PrimaryKeyBatchingStrategy, audit_events, target_details, [], null, 100, null, 2024-07-31 13:58:08.838238+00, null, gitlab_main, null, 20240729133817).

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