drm/doc: Document kapi doc expectations

We've had this already for anything new. With my drm_prime.c cleanup I
also think documentation for everything already existing is complete,
and we can bake this in as a requirements subsystem wide.

v2: Improve wording a bit (Laurent), fix typo in commit message (Sam).

Acked-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Sean Paul <sean@poorly.run>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190704145054.5701-1-daniel.vetter@ffwll.ch