drm/doc: add warning about connector_type_id stability

Mention that the connector_type_id is not stable: it depends on
driver and device probe order.

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230717131305.616855-1-contact@emersion.fr