Add missing comment in postgresql.conf.

max_logical_replication_workers requires to restart server to reflect
the new value. Per Yugo Nagata. Minor editing by me.

src/backend/utils/misc/postgresql.conf.sample

@@ -277,6 +277,7 @@
 # These settings are ignored on a publisher.
 
 #max_logical_replication_workers = 4	# taken from max_worker_processes
+					# (change requires restart)
 #max_sync_workers_per_subscription = 2	# taken from max_logical_replication_workers