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)