The mysqladmin statement suggested wasn't enough for me to pass the first database configuration stage - I kept getting a misleading bad server/user/password error. Only when I defined a database myself from within MySQL with
GRANT ALL PRIVILEGES ON wikidb.* TO 'wikidbadmin'@'localhost' IDENTIFIED BY 'strong-password';
as suggested in https://www.mediawiki.org/wiki/Manual:Maintenance_scripts MW could finally talk to MySQL. This tested with MariaDB 10.0.29 and localhost on Linux Mint.
I'm not experienced enough with Linux to tell for sure if adding this to the page would make the document much better so I won't, but maybe someone else finds this useful.