pgsql: Improve our explanation of the OVERLAPS operator.

Log Message:
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Improve our explanation of the OVERLAPS operator.
Per gripe from Frank van Vugt.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.516 -> r1.517)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.516&r2=1.517)