drm/doc: document DRM_IOCTL_MODE_CREATE_DUMB

The main motivation is to repeat that dumb buffers should not be
abused for anything else than basic software rendering with KMS.
User-space devs are more likely to look at the IOCTL docs than to
actively search for the driver-oriented "Dumb Buffer Objects"
section.

v2: reference DRM_CAP_DUMB_BUFFER, DRM_CAP_DUMB_PREFERRED_DEPTH and
DRM_CAP_DUMB_PREFER_SHADOW (Pekka)

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230821131548.269204-1-contact@emersion.fr