Clarify "reference" in filters for `docker images`

This is possibly related to (though not the same as) #1332, "docker images --filter=reference giving seemingly unpredictable results." It's a copy of issue #7618 in docker/docker.github.io which was just now closed because I'm told it belongs here. Note that #1332 and this issue date from late 2018 with no response, yet still seem to be relevant.

There's no clear explanation of how to filter based on repository or tag. We can filter by "reference," which seems to do something similar, but it doesn't seem to work intuitively: dr images -f reference=*acme* will not list images with a repository field of docker-registry.ec2-1.acme.com:5000/acme/user.

The documentation should explain what a "reference" is how it relates to the repository and tag.

Start by locating the Docker CLI documentation for docker images --filter=reference and review how repository and tag filters are described. Done means the documentation clearly defines a reference and explains how it relates to repository and tag matching, including the reported registry example.