drm/sched: Clarify docu concerning drm_sched_job_arm()

The documentation for drm_sched_job_arm() and especially
drm_sched_job_cleanup() does not make it very clear why
drm_sched_job_arm() is a point of no return, which it indeed is.

Make the nature of drm_sched_job_arm() in the docu as clear as possible.

Suggested-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250313093053.65001-2-phasta@kernel.org