https://mariadb.com/kb/en/library/percentile_disc/ & https://mariadb.com/kb/en/library/percentile_cont/ both state that these functions are:

"is an ordered set aggregate function which can also be used as a window function"

... however it doesn't appear to be possible to use them as ordered set aggregate functions. If one attempts a query "percentile_cont(fraction) WITHIN GROUP (ORDER BY sort_expression)" without an OVER clause then a "You have an error in your SQL syntax" is returned.

I suspect this might get classified as a documentation error (ie they're only supported as window functions) - however MDEV-12985 did clearly consider both use-cases.