drm/amdgpu: fix doc by clarifying sched_list definition

expand sched_list definition for better understanding.
Also fix a typo atleast -> at least

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>