Update documentation for sharing images

[Keith Berger]

The steps on this page

https://docs.openstack.org/image-guide/share-images.html#sharing-an-image

do not take into consideration an image that was created by taking a nova snapshot from a volume backed instance. When this is done the image is actually stored as a snapshot under the source volume. In that scenario, it is also necessary to transfer the cinder volume (and snapshots).

https://docs.openstack.org/cinder/latest/cli/cli-manage-volumes.html

Without transferring the cinder volume (and snapshots), an error is reported about missing volume when an instance is launched from this shared image.

Block Device Mapping is Invalid: failed to get snapshot xxxxxxx-xxxxxx-xxxxx-xxxxxx. (HTTP 400) (Request-ID: req-xxxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx)

[Erno Kuvaja (jokke)]

We indeed should at least document the expectations of the behavior.

[Alexandra Settle (alexandra-settle)]

I can take this, Erno :) once I get a mo this week I'll work with Keith to update!