Important topic missing from docs?

I’ve been following the Getting Started docs and there seems to be a very important step missing.

This page requires you to set up a docker repo or you’ll get a very unhelpful:

$ docker push banksysan/docker-whale
The push refers to a repository [] (len: 1)
654156f34320: Preparing
unauthorized: access to the requested resource is not authorized

A discussion on what a repo and what a namespace is is needed. It looks like Docker repos are nothing liek Git repos (which confused me, and still does).