drm/sched: Document what the timedout_job method should do

The documentation is a bit vague and doesn't really describe what the
->timedout_job() is expected to do. Let's add a few more details.

v5:
* New patch

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210630062751.2832545-2-boris.brezillon@collabora.com