Doc: mention CREATE+ATTACH PARTITION with CREATE TABLE...PARTITION OF.

Doc: mention CREATE+ATTACH PARTITION with CREATE TABLE...PARTITION OF.

Clarify that ATTACH/DETACH PARTITION can be used to perform partition
maintenance with less locking than straight CREATE TABLE/DROP TABLE.
This was already stated in some places, but not emphasized.

Back-patch to v14 where DETACH PARTITION CONCURRENTLY was added.
(We had lower lock levels for ATTACH PARTITION before that, but
this wording wouldn't apply.)

Justin Pryzby, reviewed by Robert Treat and Jakub Wartak;
a little further wordsmithing by me

Discussion: https://postgr.es/m/20220718143304.GC18011@telsasoft.com