pgsql: Add documentation reference to statistical functions from

Log Message:
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Add documentation reference to statistical functions from func.sgml.

Jim Nasby

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.380 -> r1.381)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.380&r2=1.381)