doc: Improve description of publish_via_partition_root

doc: Improve description of publish_via_partition_root

Reword publish_via_partition_root's opening paragraph. Describe its
behavior more clearly, and directly state that its default is false.

Per complaint by Peter Smith; final text of the patch made in
collaboration with Chao Li.

Author: Chao Li <li.evan.chao@gmail.com>
Author: Peter Smith <peter.b.smith@fujitsu.com>
Reported-by: Peter Smith <peter.b.smith@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAHut%2BPu7SpK%2BctOYoqYR3V4w5LKc9sCs6c_qotk9uTQJQ4zp6g%40mail.gmail.com
Backpatch-through: 14