This allows us to set things like dependencies in a way that we can
catch typos at eval time.
So instead of
```nix
systemd.services.foo.wants = [ "bar.service" ];
```
we can write
```nix
systemd.services.foo.wants = [ config.systemd.services.bar.name ];
```
which will throw an error if no such service has been defined.
Not all cases can be done like this (eg template services), but in a lot
of cases this will allow to avoid typos.
There is a matching option on the unit option
(`systemd.units."foo.service".name`) as well.
conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running
nix-doc-munge nixos/**/*.nix
nix-doc-munge --import nixos/**/*.nix
the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.
