The NixOS variant of Markdown doesn't make a distinction between
`<code>` and `<literal>` or `<quote>` and... quotes, and doesn't
support `<parameter>` or `<replaceable>`. These are infrequently used
(apart from `<code>`) and don't add much, so just convert them to
simpler forms to allow the options containing them to be converted
to Markdown automatically.
A few minor syntactic adjustments were also made to make
`nix-doc-munge`'s job easier.
`nix-doc-munge` can't handle these, which is understandable as I can
barely handle them either. There are a few infelicities here: the
current processor can't handle multiple terms to one description in
a description list so they get comma-separated in one case, and one
case that should ideally render as a `<figure>` with a `<figcaption>`
in HTML is reduced to a paragraph with some `<strong>` text. (Which, in
fairness, is how it rendered in practice with the DocBook anyway.) The
docs generator has since been updated to handle figures, but we can't
use it until moving off DocBook output.
