First pass at replacing parent-child metaphor to describe docker links.

Addresses issue #7141

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)

a little git by
Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: SvenDowideit)