Doc: Improve description around ALTER TABLE ATTACH PARTITION

This clarifies more how to use and how to take advantage of constraints
when attaching a new partition.

Author: Justin Pryzby
Reviewed-by: Amit Langote, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20191028001207.GB23808@telsasoft.com
Backpatch-through: 10