pgsql: Reword fsync and full_page_writes docs to be clearer about when

Log Message:
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Reword fsync and full_page_writes docs to be clearer about when to turn
them off.

Josh Berkus, with slight wording changes by me.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.279 -> r1.280)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.279&r2=1.280)