Docs for marking a batched BG migration finished

What does this MR do and why?

This MR adds a section for force-marking a BG migration finished in case it's broken.

Related to: #393216 (closed)

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.

doc/update/background_migrations.md

@@ -336,19 +336,19 @@ When the background migration is determined to be "safe" to skip, the migration
 WARNING:
 Make sure you create a backup before proceeding.
 
-```shell
+```ruby
 # Start the rails console
 
 connection = ApplicationRecord.connection # or Ci::ApplicationRecord.connection, depending on which DB was the migration scheduled
 
 Gitlab::Database::SharedModel.using_connection(connection) do
-  migration = Gitlab::Database::BackgroundMigration::BatchedMigration.for_configuration(
-    :gitlab_main, # or :gitlab_ci
+  migration = Gitlab::Database::BackgroundMigration::BatchedMigration.find_for_configuration(
+    Gitlab::Database.gitlab_schemas_for_connection(connection),
     'BackfillUserDetailsFields',
     :users,
     :id,
     []
-  ).first!
+  )
 
   # mark all jobs completed
   migration.batched_jobs.update_all(status: Gitlab::Database::BackgroundMigration::BatchedJob.state_machine.states['succeeded'].value)