feat(tests): add optional db-sync check docstring

- Added a docstring entry for the optional db-sync check in the
  `TestBuildLocking` class in `test_spend_build.py`.

Author: mkoura

cardano_node_tests/tests/tests_plutus_v2/test_spend_build.py

@@ -65,6 +65,7 @@ def test_txout_locking(
         * create the necessary Tx outputs
         * spend the locked UTxO
         * check that the expected UTxOs were correctly spent
+        * (optional) check transactions in db-sync
         """
         temp_template = common.get_test_id(cluster)
         script_fund = 10_000_000