drm: document that blobs are ref'counted

User-space doesn't need to keep track of blobs that might be in use by
the kernel. User-space can just destroy blobs as soon as they don't need
them anymore.

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/wgav99DTGfubfVPiurrydQEiyufYpxlJQZ0wJMWYBQ@cp7-web-042.plabs.ch