These help `nix-munge-doc` automate more of the Markdown conversion
process. See the following nixpkgs commits for explanations of many
of these changes:
* 275a34e0d8
* 694d5b19d3
* f1d39b6d61
* 16102dce2f
I couldn't think of any particularly good way to format the
`system.defaults` breadcrumbs, so I just made them standalone
paragraphs. They weren't rendering correctly in DocBook anyway.
b97c235e37