chore(sqltest): update readme (#6228)

internal/db/sqltest/README.md

@@ -3,7 +3,9 @@
 This test suite is used to test behavior of the database logic.
 In particular the data warehouse implementation is
 completely in sql via plpgsql, functions, and triggers.
-This test suite is also implemented directly in sql.
+Note that these tests are not limited to testing triggers - they should also be used to test constraints and indexes.
+
+This test suite is implemented directly in sql.
 
 ## Organization