PG docs - CREATE PUBLICATION publish_via_partition_root

The PG docs are not quite as clear as they might be regarding the
default value of the "partition_via_partition_root" option.

This small change originated from the row-filter thread [RF], but then
it was decided it should really be a separate patch (hence this post).

[Tomas 11/7] comment 2: "Why is the patch changing
publish_via_partition_root docs? That seems like a rather unrelated
bit."
[Peter 13/7] comment 4: "IMO you need to propose this one in another thread"

PSA.

-------
[RF]
https://www.postgresql.org/message-id/flat/CAHE3wggb715X%2BmK_DitLXF25B%3DjE6xyNCH4YOwM860JR7HarGQ%40mail.gmail.com
[Tomas 11/7] https://www.postgresql.org/message-id/849ee491-bba3-c0ae-cc25-4fce1c03f105%40enterprisedb.com
[Peter 13/7]
https://www.postgresql.org/message-id/CAHut%2BPsjWbBHTT1dS%3Dji%3DPcziQU1%2BmYqh6G0gFG6AGTCMhTN_g%40mail.gmail.com

Kind Regards,
Peter Smith.
Fujitsu Australia