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

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/988612c9c27f8f62bcd8a5e30f3b57e883189308

Modified Files
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doc/src/sgml/ref/create_publication.sgml | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)