Information about EXCEPT ALL is inconsistent between KB, JIRA, code and tests

It is unclear from the available sources whether EXCEPT ALL is supposed to work at all, and if it is, to what extent.

The KB says that is is not supported:

ALL is not supported by EXCEPT (and it is difficult to make sense of ALL with EXCEPT).

MDEV-10140 in scope of which EXCEPT was implemented says that ALL is supported, and explains the semantics (however strange it is):

With ALL, a row that has m duplicates in the left table and n duplicates in the right table will appear max(m-n,0) times in the result set.

Also, when EXCEPT ALL is executed in current pre-10.3.1 server, it throws an error, but it's not "unsupported":

MariaDB [test]> select * from t1 except all select * from t2;

ERROR 1221 (HY000): Incorrect usage of EXCEPT and ALL

So,

• if EXCEPT ALL is not supported, I think it's better to throw the corresponding error message rather than Incorrect usage;
• please reconcile KB with JIRA so they both contain correct information.