drm/sched: drm_sched_job_cleanup(): correct false doc

drm_sched_job_cleanup()'s documentation claims that calling
drm_sched_job_arm() is a "point of no return", implying that afterwards
a job cannot be cancelled anymore.

This is not correct, as proven by the function's code itself, which
takes a previous call to drm_sched_job_arm() into account. In truth, the
decisive factors are whether fences have been shared (e.g., with other
processes) and if the job has been submitted to an entity already.

Correct the wrong docstring.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250304141346.102683-2-phasta@kernel.org