doc:  show how interval's 3 unit buckets behave using EXTRACT()
doc:  show how interval's 3 unit buckets behave using EXTRACT()

This clarifies when justify_days() and justify_hours() are useful.
Paragraph moved too.

Reported-by: vodevsh@gmail.com

Discussion: https://postgr.es/m/152698651482.26744.15456677499485530703@wrigleys.postgresql.org

Backpatch-through: 9.3

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