Add maintenance_io_concurrency to postgresql.conf.sample.

New GUC from commit fc34b0d.

Author: macdice

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

@@ -174,6 +174,7 @@
 # - Asynchronous Behavior -
 
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching
+#maintenance_io_concurrency = 10	# 1-1000; 0 disables prefetching
 #max_worker_processes = 8		# (change requires restart)
 #max_parallel_maintenance_workers = 2	# taken from max_parallel_workers
 #max_parallel_workers_per_gather = 2	# taken from max_parallel_workers