drm/atomic: Make the kerneldoc a bit clearer

Crank up the warning a notch and point at the right set of locking
functions for atomic drivers.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201002075620.4157591-1-daniel.vetter@ffwll.ch