pgsql: Add section on reliable operation, talking about caching and

Log Message:
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Add section on reliable operation, talking about caching and storage
subsystem reliability.

Modified Files:
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pgsql/doc/src/sgml:
wal.sgml (r1.31 -> r1.32)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/wal.sgml.diff?r1=1.31&r2=1.32)