drm/dp_mst: Improve kdocs for drm_dp_check_act_status()

No functional changes.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Reviewed-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406221253.1307209-2-lyude@redhat.com