drm/doc: Document expectation that userspace review looks at kernel uAPI.

The point of this review process is that userspace using the new uAPI
can actually live with the uAPI being provided, and it's hard to know
that without having actually looked into a kernel patch yourself.

Signed-off-by: Eric Anholt <eric@anholt.net>
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190424185617.16865-2-eric@anholt.net


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>