drm/doc: device hot-unplug for userspace

Set up the expectations on how hot-unplugging a DRM device should look like to
userspace.

Written by Daniel Vetter's request and largely based on his comments in IRC and
from https://lists.freedesktop.org/archives/dri-devel/2020-May/265484.html .

A related Wayland protocol change proposal is at
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/35



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Simon Ser <contact@emersion.fr>
Cc: Ben Skeggs <skeggsb@gmail.com>
Cc: Karol Herbst <kherbst@redhat.com>
Acked-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707113805.30936-1-ppaalanen@gmail.com