Add checkpoint_warning to warn of excessive checkpoints caused by too

few WAL files.

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

@@ -65,6 +65,7 @@
 #
 #checkpoint_segments = 3	# in logfile segments, min 1, 16MB each
 #checkpoint_timeout = 300	# range 30-3600, in seconds
+#checkpoint_warning = 30	# 0 is off, in seconds
 #
 #commit_delay = 0		# range 0-100000, in microseconds
 #commit_siblings = 5		# range 1-1000