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? 
@lsm5 :( OK, I don't know what to do here. Either we have some ${DOCKER_OPTS} environment variable to the service file and extend the ExecStart line to take that (not ideal!) or we need some way of appending multiple flags via env variables.