drm/scheduler: Improve documentation

Function drm_sched_entity_push_job() doesn't have a return value,
remove the return value description for it.
Correct several other typo errors.

v2 (Philipp):
- more correction with related comments.

Signed-off-by: Shuicheng Lin <shuicheng.lin@intel.com>
Reviewed-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20240917144732.2758572-1-shuicheng.lin@intel.com