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)
Just noticed this and I'd like to double check:
Shouldn't "for the source" be "from the source", i.e., "from the source to the recipient"?