Update database migration docs to avoid rubocop

The suggested approach in this example would require the addition of a
rubocop disable flag. We can just use the helper method instead, as the
`with_lock_retries` functionality is built into it.

MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45157

doc/development/migration_style_guide.md

@@ -295,13 +295,16 @@ end
 
 Adding foreign key to `projects`:
 
+We can use the `add_concurrenct_foreign_key` method in this case, as this helper method
+has the lock retries built into it.
+
 ```ruby
 include Gitlab::Database::MigrationHelpers
 
+disable_ddl_transaction!
+
 def up
-  with_lock_retries do
-    add_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade
-  end
+  add_concurrent_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade
 end
 
 def down
@@ -316,10 +319,10 @@ Adding foreign key to `users`:
 ```ruby
 include Gitlab::Database::MigrationHelpers
 
+disable_ddl_transaction!
+
 def up
-  with_lock_retries do
-    add_foreign_key :imports, :users, column: :user_id, on_delete: :cascade
-  end
+  add_concurrent_foreign_key :imports, :users, column: :user_id, on_delete: :cascade
 end
 
 def down