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)
@fredlf Sometimes it becomes quite easy to be fascinated by something rather simple (and relatively clear to the most) and end up having a long and hard - and painful ;-) - think about it. This is one of those! I will see about getting the book and reading these theories re reader expectation - sounds intriguing. Thanks!

Still feels odd to refer to two particular containers, to describe an expected and usual behavior that applies to any and all [containers] in *simple present* (as there is no peculiarity), but hey ho ;-)

Your point about this theory is well understood .. cheerio!