Revert off setting mention for client_min_messages.

Set log_min_error_messages to the proper and agreed-upon default, PANIC
(off).  (BACKPATCH)

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

@@ -121,8 +121,7 @@
 				#   panic
 #client_min_messages = notice	# Values, in order of decreasing detail:
 				#   debug5, debug4, debug3, debug2, debug1,
-				#   log, info, notice, warning, error, 
-				#   panic(off)
+				#   log, info, notice, warning, error
 #silent_mode = false
 
 #log_connections = false