Document regression in pulling aliases

Docker pull currently doesn’t pull all aliases of an image. This is a
(temporary?) regression caused by changes in the V2 registry, as
described in https://github.com/docker/docker/issues/8689

This change adds a note to the documentation about this regression.

Signed-off-by: Sebastiaan van Stijn <thaJeztah@users.noreply.github.com>