doc:  adjust order of NUMERIC arguments to match syntax
doc:  adjust order of NUMERIC arguments to match syntax

Specifically, mention precision before scale

Reported-by: claytonjsalem@gmail.com

Discussion: https://postgr.es/m/152967566691.1268.1062965601465200209@wrigleys.postgresql.org

Backpatch-through: 9.3

adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb