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