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_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/591128f9c9c0e0b9c511c5eba6f889bd1daf3c4c

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