drm/doc: Document legacy_cursor_update better

It's the horror and shouldn't be used. Realized we're not clear on
this in a discussion with Rob about what msm is doing to better
support async commits.

v2: Refine existing todo item to include this (Thomas)

Cc: Sean Paul <sean@poorly.run>
Cc: Rob Clark <robdclark@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>
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/20201023123925.2374863-3-daniel.vetter@ffwll.ch