Improve postgresql.conf.sample comments about parallel workers.

David Rowley, reviewed by Amit Kapila

Discussion: http://postgr.es/m/CAKJS1f8gPEUPscj6kSqpveMnnx9_3ZypzwsKstv+8atx6VmjBg@mail.gmail.com

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

@@ -161,8 +161,9 @@
 
 #effective_io_concurrency = 1		# 1-1000; 0 disables prefetching
 #max_worker_processes = 8		# (change requires restart)
-#max_parallel_workers_per_gather = 2	# taken from max_worker_processes
-#max_parallel_workers = 8	    # total maximum number of worker_processes
+#max_parallel_workers_per_gather = 2	# taken from max_parallel_workers
+#max_parallel_workers = 8	    # maximum number of max_worker_processes that
+					# can be used in parallel queries
 #max_logical_replication_workers = 4	# taken from max_worker_processes
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 					# (change requires restart)