Add vacuum_defer_cleanup_age to postgresql.conf.sample.

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

@@ -187,6 +187,7 @@
 #recovery_connections = on	# allows connections during recovery
 #max_standby_delay = 30		# max acceptable standby lag (s) to allow queries
 				# to complete without conflict; -1 disables
+#vacuum_defer_cleanup_age = 0 # num transactions by which cleanup is deferred
 
 # - Replication -