`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.
Before, loading a module would be guarded by an optional platform
condition. This made it possible to avoid loading and evaluating a
module if it did not support the host platform.
Unfortunately, this made it impossible to share a single configuration
between GNU/Linux and Darwin hosts, which some wish to do.
This removes the conditional load and instead inserts host platform
assertions in the modules that are platform specific.
Fixes#1906
