parallel_schedule: add comment on event_trigger test dependency

This is cosmetic, so no backpatch.

Discussion: https://postgr.es/m/20230405233356.qs4w4jtfc3kq4obl@alvherre.pgsql
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>

Author: alvherre

src/test/regress/parallel_schedule

@@ -121,7 +121,8 @@ test: plancache limit plpgsql copy2 temp domain rangefuncs prepare conversion tr
 # ----------
 test: partition_join partition_prune reloptions hash_part indexing partition_aggregate partition_info tuplesort explain compression memoize stats
 
-# event_trigger cannot run concurrently with any test that runs DDL
+# event_trigger depends on create_am and cannot run concurrently with
+# any test that runs DDL
 # oidjoins is read-only, though, and should run late for best coverage
 test: event_trigger oidjoins