pgsql: Fix documentation & config inconsistencies around 428b1d6b2.

Fix documentation & config inconsistencies around 428b1d6b2.

Several issues:
1) checkpoint_flush_after doc and code disagreed about the default
2) new GUCs were missing from postgresql.conf.sample
3) Outdated source-code comment about bgwriter_flush_after's default
4) Sub-optimal categories assigned to new GUCs
5) Docs suggested backend_flush_after is PGC_SIGHUP, but it's PGC_USERSET.
6) Spell out int as integer in the docs, as done elsewhere

Reported-By: Magnus Hagander, Fujii Masao
Discussion: CAHGQGwETyTG5VYQQ5C_srwxWX7RXvFcD3dKROhvAWWhoSBdmZw(at)mail(dot)gmail(dot)com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8f91d87d43d021db92c6edd966a4bb8c3a81ae39

Modified Files
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doc/src/sgml/config.sgml | 10 +++---
src/backend/utils/misc/guc.c | 50 +++++++++++++--------------
src/backend/utils/misc/postgresql.conf.sample | 6 ++++
3 files changed, 35 insertions(+), 31 deletions(-)