The copyright owner can license under whatever license they like. As many have said even before it was sold to Sun all contributions needed a copyright assignm

The copyright owner can license under whatever license they like. As many have said even before it was sold to Sun all contributions needed a copyright assignment before they were accepted (so they could sell the option to make closed source modifications amongst other things) so Oracle can now license as they wish. What they can't do is revoke the existing GPL licenses so you can still develop and modify the old docs. Having said that this is Oracle and you should probably just run for the hills (MariaDB) before Oracle make it hard to make it fully compatible with future version of MySQL by doing something else.