pgsql: Improve WAL reliability documentation, and add more cross-refere

Improve WAL reliability documentation, and add more cross-references to it.

In particular, we are now more explicit about the fact that you may need
wal_sync_method=fsync_writethrough for crash-safety on some platforms,
including MaxOS X. There's also now an explicit caution against assuming
that the default setting of wal_sync_method is either crash-safe or best
for performance.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=c1db5296429a8f26929c1afe5c9d217ba65a3b53

Modified Files
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doc/src/sgml/config.sgml | 6 +++++-
doc/src/sgml/wal.sgml | 11 ++++++++---
2 files changed, 13 insertions(+), 4 deletions(-)