pgsql: Document the interaction of write-barrier-enabled file systems,

Log Message:
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Document the interaction of write-barrier-enabled file systems, and BBU
caches, per June email thread.

Modified Files:
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pgsql/doc/src/sgml:
wal.sgml (r1.66 -> r1.67)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/wal.sgml?r1=1.66&r2=1.67)