most of these are hidden because they're either part of a submodule that
doesn't have its type rendered (eg because the submodule type is used in
an either type) or because they are explicitly hidden. some of them are
merely hidden from nix-doc-munge by how their option is put together.
This fixes a serious bug on NixOS with swraid where mdadm arrays weren't
properly stopped on shutdown. Rather than fixing the unit by adding
`Before=final.target` we completely move to upstream units, which uses
systemd shutdown hooks instead. This also drives down maintenance costs
for us.
Also add required systemd services for starting/stopping mdmon.
Closes#13447.
abbradar: fixed `mdadmShutdown` service name according to de facto conventions.
