pgsql: Clarify NULLIF example found by "Vesa-Matti J Kari"

Log Message:
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Clarify NULLIF example

found by "Vesa-Matti J Kari" <vmkari@cc.helsinki.fi>

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.482 -> r1.483)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.482&amp;r2=1.483)