pgsql: Doc: improve documentation of the JSON object aggregates.

Doc: improve documentation of the JSON object aggregates.

Say which value wins for a repeated key, and how skipped null values
interact with that and with the unique check. Also point out that
duplicate keys behave differently for json and jsonb cases. (I still
think that's a bug, but at least now it's documented.)

Author: shihao zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAGRkXqTCfSu=aEUCvKDTGroEWOcyz3MCv4Xi8x1PZ1T1rEmG=Q@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_17_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/298191036cce3b7634fd4d4b71eeb29def103f0b

Modified Files
--------------
doc/src/sgml/func.sgml | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)