add_description_unit has been deprectated for a really long time, and was
finally removed (seemingly in Sphinx 2.0, but this doesn't seem to be
properly documented anywhere I can find), so we shouldn't be using this
any more.
Anyway, let's update the code.
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5691>
7f5061c0b7