I think I would swap this around and describe the change to the config first, and then the prefixing of the relative urls. I say this because when I was reading it, I got confused by the reference to the config variable being so close to the command line flag. Perhaps something like the quoted text below?

> To take advantage of this feature, first add the production `baseurl` to your `_config.yml` file for Jekyll your site. Then, throughout the site, simply prefix relative urls with `{{ site.baseurl }}`. When you're ready to preview your site locally, pass along the `--baseurl` flag to `jekyll serve` and Jekyll will swap in whatever you've passed along, ensuring your links remain working in both environments