drm/sched: Document purpose of drm_sched_{start,stop}

drm_sched_start()'s and drm_sched_stop()'s names suggest that those
functions might be intended for actively starting and stopping the
scheduler on initialization and teardown.

They are, however, only used on timeout handling (reset recovery). The
docstrings should reflect that to prevent confusion.

Document those functions' purpose.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241029133819.78696-2-pstanner@redhat.com