The old open-source driver for AMD/ATI GPUs is commonly known as "radeon"
despite the historical package name xf86-video-ati. For example it presents
itself as RADEON in the Xorg log. So adding "radeon" to videoDrivers should
work.
Also changed the docs for the videoDrivers option to use "radeon" in the
default value instead of "ati".
Fixes#37917