pgsql: Document strange jsonb sort order for empty top level arrays

Document strange jsonb sort order for empty top level arrays

Slightly faulty logic in the original jsonb code (commit d9134d0a355)
results in an empty top level array sorting less than a json null. We
can't change the sort order now since it would affect btree indexes over
jsonb, so document the anomaly.

Backpatch to all live branches (13 .. 17)

In master, also add a code comment noting the anomaly.

Reported-by: Yan Chengpen
Reviewed-by: Jian He

Discussion: https://postgr.es/m/OSBPR01MB45199DD8DA2D1CECD50518188E272@OSBPR01MB4519.jpnprd01.prod.outlook.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bd606ed8ec65f7ad388c3dead9ba200bf0b4fa6a

Modified Files
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doc/src/sgml/json.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)