[Maria-docs] GA should also mean "documented" :)

It's great to see that 10.2 is now GA. But from a quick look, I've seen that documentation is incomplete: * Recursive CTE not documented: https://mariadb.com/kb/en/mariadb/common-table-expressions/ * Window functions: "this article is currently incomplete" https://mariadb.com/kb/en/mariadb/window-functions-overview/ There could be other undocumented features, these are the ones I've found immediately. Please, fill those gaps so we can test the features. Regards, Federico