Update replaceable_schema.sql

replaceable_schema.sql

@@ -4,6 +4,9 @@
 -- except for these things which are associated with something other than a schema (usually a table):
 --   * A trigger if the function name after `EXECUTE FUNCTION` is in `r` (dropping `r` drops the trigger)
 --
+-- `r` is not temporarily set to the default schema because it would not affect some things (such as triggers) which
+-- makes it hard to tell if the rule above is being followed.
+--
 -- If you add something in `r` that depends on something (such as a table) created in a new migration, then down.sql must use
 -- `CASCADE` when dropping it. This doesn't need to be fixed in old migrations because the "transactions-without-migrations"
 -- migration runs `DROP SCHEMA IF EXISTS r CASCADE` in down.sql.