drm: document drm_mode_get_property

It's not obvious what the fields mean and how they should be used.
The most important detail is the link to drm_property.flags, which
describes how property types work.

v2: document enum drm_mode_property_enum, add ref to "Modeset Base
Object Abstraction" (Daniel)

Signed-off-by: Simon Ser <contact@emersion.fr>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210802072826.500078-1-contact@emersion.fr