Extract the systemd docs from various places and add a little more

@lsm5 @philips @jfrazelle I don't grok why the fedora `docker.services` file is different from the one in https://github.com/docker/docker/blob/master/contrib/init/systemd

if you're ok with it, I'll update the one in our repo - or maybe we should delete them? 
We need to flip this sentence so the context comes first, something like: 
If you need to add an HTTP Proxy, set a different directory or partition for the Docker runtime files, or make other customizations, read our systemd article to learn how to [customize your systemd Docker daemon options](/articles/systemd/).