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)
I understand that's the intention. It just feels bit uncanny put this way.
Because linking *any* container will do the same (i.e., what's being explained in this doc), not just these two.

If we were talking about a particular container or two, which behave differently from the rest [in terms of linking] and if that was what's trying to be be demonstrated here, I think this would work. Linking containers, however, is almost a universal thingy, with an expected, general behavior that does not only apply to these two.

Furthermore, a couple sentences later, we say "in our example .." and bring back up the context, which should - and does - serve for the purpose IMO.