Proposal: Copy data from container to empty vol in all cases

Currently data is only copied into a volume if it is a _normal_ volume,
e.g. not for volumes-from or host-mounts.

At one point a host-mounted volume would not be automatically created if
the dir didn't exist already, today that is no longer true.  If we are
willing to create the dir, we might as well populate it too.

This makes it so data is always copied if the volume is empty.
I think this would be clearer if we present the info in a different order, with the contextual information coming first. We should also be explicit about volumes NOT being reinitialized, because it seems a little bit counter-intuitive. I suggest something like: 
Previously, when creating a volume linked to a container via `VolumesFrom` or `Binds`, data from the container at the specified path would not be copied. Now, as long as the volume is empty, data from the container will always be copied when the volume is created. Note that `docker restart` will not re-initialize the linked volume.