pgsql: Bring some clarity to the defaults for the xxx_flush_after param

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>

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/255bcd27f635a1b9682e7e0dfd348b0f8b268df1

Modified Files
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doc/src/sgml/config.sgml                      | 33 ++++++++++++++-------------
src/backend/access/transam/xlog.c             |  2 +-
src/backend/utils/misc/guc.c                  |  8 +++----
src/backend/utils/misc/postgresql.conf.sample | 12 ++++------
src/bin/initdb/initdb.c                       | 21 +++++++++++++++++
src/include/pg_config_manual.h                | 18 +++++++++++++++
src/include/storage/bufmgr.h                  | 11 ---------
7 files changed, 65 insertions(+), 40 deletions(-)