Bring some clarity to the defaults for the xxx_flush_after parameters.

Instead of confusingly stating platform-dependent defaults for these
parameters in the comments in postgresql.conf.sample (with the main
entry being a lie on Linux), teach initdb to install the correct
platform-dependent value in postgresql.conf, similarly to the way
we handle other platform-dependent defaults.  This won't do anything
for existing 9.6 installations, but since it's effectively only a
documentation improvement, that seems OK.

Since this requires initdb to have access to the default values,
move the #define's for those to pg_config_manual.h; the original
placement in bufmgr.h is unworkable because that file can't be
included by frontend programs.

Adjust the default value for wal_writer_flush_after so that it is 1MB
regardless of XLOG_BLCKSZ, conforming to what is stated in both the
SGML docs and postgresql.conf.  (We could alternatively make it scale
with XLOG_BLCKSZ, but I'm not sure I see the point.)

Copy-edit related SGML documentation.

Fabien Coelho and Tom Lane, per a gripe from Tomas Vondra.

Discussion: <30ebc6e3-8358-09cf-44a8-578252938424@2ndquadrant.com>

src/backend/utils/misc/guc.c

@@ -2281,7 +2281,6 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS
 		},
 		&checkpoint_flush_after,
-		/* see bufmgr.h: OS dependent default */
 		DEFAULT_CHECKPOINT_FLUSH_AFTER, 0, WRITEBACK_MAX_PENDING_FLUSHES,
 		NULL, NULL, NULL
 	},
@@ -2310,12 +2309,12 @@ static struct config_int ConfigureNamesInt[] =
 
 	{
 		{"wal_writer_flush_after", PGC_SIGHUP, WAL_SETTINGS,
-			gettext_noop("Amount of WAL written out by WAL writer triggering a flush."),
+			gettext_noop("Amount of WAL written out by WAL writer that triggers a flush."),
 			NULL,
 			GUC_UNIT_XBLOCKS
 		},
 		&WalWriterFlushAfter,
-		128, 0, INT_MAX,
+		(1024*1024) / XLOG_BLCKSZ, 0, INT_MAX,
 		NULL, NULL, NULL
 	},
 
@@ -2439,7 +2438,6 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS
 		},
 		&bgwriter_flush_after,
-		/* see bufmgr.h: OS dependent default */
 		DEFAULT_BGWRITER_FLUSH_AFTER, 0, WRITEBACK_MAX_PENDING_FLUSHES,
 		NULL, NULL, NULL
 	},
@@ -2467,7 +2465,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BLOCKS
 		},
 		&backend_flush_after,
-		0, 0, WRITEBACK_MAX_PENDING_FLUSHES,
+		DEFAULT_BACKEND_FLUSH_AFTER, 0, WRITEBACK_MAX_PENDING_FLUSHES,
 		NULL, NULL, NULL
 	},
 

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

@@ -156,8 +156,7 @@
 #bgwriter_delay = 200ms			# 10-10000ms between rounds
 #bgwriter_lru_maxpages = 100		# 0-1000 max buffers written/round
 #bgwriter_lru_multiplier = 2.0		# 0-10.0 multiplier on buffers scanned/round
-#bgwriter_flush_after = 0		# 0 disables,
-					# default is 512kB on linux, 0 otherwise
+#bgwriter_flush_after = 0		# measured in pages, 0 disables
 
 # - Asynchronous Behavior -
 
@@ -166,7 +165,7 @@
 #max_parallel_workers_per_gather = 2	# taken from max_worker_processes
 #old_snapshot_threshold = -1		# 1min-60d; -1 disables; 0 is immediate
 					# (change requires restart)
-#backend_flush_after = 0		# 0 disables, default is 0
+#backend_flush_after = 0		# measured in pages, 0 disables
 
 
 #------------------------------------------------------------------------------
@@ -196,7 +195,7 @@
 #wal_buffers = -1			# min 32kB, -1 sets based on shared_buffers
 					# (change requires restart)
 #wal_writer_delay = 200ms		# 1-10000 milliseconds
-#wal_writer_flush_after = 1MB		# 0 disables
+#wal_writer_flush_after = 1MB		# measured in pages, 0 disables
 
 #commit_delay = 0			# range 0-100000, in microseconds
 #commit_siblings = 5			# range 1-1000
@@ -207,8 +206,7 @@
 #max_wal_size = 1GB
 #min_wal_size = 80MB
 #checkpoint_completion_target = 0.5	# checkpoint target duration, 0.0 - 1.0
-#checkpoint_flush_after = 0		# 0 disables,
-					# default is 256kB on linux, 0 otherwise
+#checkpoint_flush_after = 0		# measured in pages, 0 disables
 #checkpoint_warning = 30s		# 0 disables
 
 # - Archiving -