pgsql: Add mention of using tools/fsync to test fsync methods. Restruct

Add mention of using tools/fsync to test fsync methods. Restructure
recent wal_sync_method doc paragraph to be clearer.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=6f609199df2c8d38ed0391180a2ff467333ae57c

Modified Files
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doc/src/sgml/config.sgml | 12 ++++++------
doc/src/sgml/wal.sgml | 12 +++++++-----
2 files changed, 13 insertions(+), 11 deletions(-)