drm/atomic-helper: document drm_atomic_helper_check() restrictions

The drm_atomic_helper_check() calls drm_atomic_helper_check_modeset()
insternally. Document that corresponding restrictions also apply to the
drivers that call the former function (as it's easy to miss the
documentation for the latter function).

Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241222-drm-dirty-modeset-v1-1-0e76a53eceb9@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>