pgsql: Document that Datestyle no longer controls interval output, per

Log Message:
-----------
Document that Datestyle no longer controls interval output, per
suggestion from Ron Mayer

Modified Files:
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pgsql/doc/src/sgml:
release.sgml (r1.598 -> r1.599)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.598&r2=1.599)