Explain links in a bit more detail

Fixes #689 

Not sure this is perfect, but hopefully an improvement on what we had before. It would've helped massively if Docker's own documentation on container linking hadn't shown an example with incorrect environment variable names, something I have also opened a PR for:

https://github.com/docker/docker/pull/8008

(for a while I got confused, thinking I was fundamentally misunderstanding something about how environment variables were named)
Or perhaps change to something like "As demonstrated in the example above"....?
Not sure about the phrasing, we could drop it entirely as per your comment, but my aim in writing it is to draw people's attention back to the example config if they're not understanding how this is different to Docker's native linking behaviour.