pgsql: Add some real descriptions to the multiargument aggregate

Log Message:
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Add some real descriptions to the multiargument aggregate functions rather
than just showing the incomprehensible formulas.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.344 -> r1.345)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.344&r2=1.345)