pgsql: Document atan2() as y/x, rather than x/y, to conform to general

Log Message:
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Document atan2() as y/x, rather than x/y, to conform to general usage.

Andrew Maclean

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.394 -> r1.395)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.394&r2=1.395)