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? 
@philips sorry, when you say adding DOCKER_OPTS/OPTIONS to the service file, do you mean it'll be set in the service file itself, or will it live in a place like `/etc/sysconfig/docker` which will survive package updates? The latter is what fedora and related distros do atm.