Document possible transaction_if_not_already error

This function can raise an error, but that was not previously documented.

Co-authored-by: Lily <code@lilyf.org>

Author: meshy

docs/reference/settings.md

@@ -35,9 +35,10 @@ on a per-test basis.
 (default: `True`)
 
 [`transaction`][django_subatomic.db.transaction]
+and [`transaction_if_not_already`][django_subatomic.db.transaction_if_not_already]
 will raise `subatomic.db._UnhandledCallbacks` in tests
-if it detects any lingering unhandled after-commit callbacks
-when it's called.
+if they detect any lingering unhandled after-commit callbacks
+when they are called.
 [`part_of_a_transaction`][django_subatomic.test.part_of_a_transaction]
 will raise `subatomic.test._UnhandledCallbacks` instead.
 Note: because these exceptions each represent a programming error,