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/access/transam/xlog.c

@@ -2759,7 +2759,7 @@ XLogFlush(XLogRecPtr record)
  * This routine is invoked periodically by the background walwriter process.
  *
  * Returns TRUE if there was any work to do, even if we skipped flushing due
- * to wal_writer_delay/wal_flush_after.
+ * to wal_writer_delay/wal_writer_flush_after.
  */
 bool
 XLogBackgroundFlush(void)