Hello,

Writing properties, I often hesitate between using types `HostName` or `Domain` for the FQDN of a machine.
This is not 'very important' since both are "just" `String`, but the type carries semantics and I'd rather keep consistent (helps understanding the code better).

Here are the docs :

http://hackage.haskell.org/package/propellor-5.9.1/docs/Propellor-Types-OS.html#t:HostName
http://hackage.haskell.org/package/propellor-5.9.1/docs/Propellor-Types-Dns.html#t:Domain

So `HostName` documentation to me looks like really corresponding to a machine's FQDN, but may also be the IP of the machine.
Conversely `Domain` is not documented (in its module) but it is used in the 'domain part of the FQDN' in some modules; eg. in Propellor/Property/Hostname.hs
Even clearer is the `Propellor.Property.DNS` module in which I clearly understand the choice of `Domain` vs. `HostName`.

Still it seems to me that sometimes one sees `Domain` where a `HostName` would be expected. One such example is in `LetsEncrypt`
Maybe I am just to confused by a few places where `Domain` is used while I would (maybe wrongly) expect `HostName` ?

What am I missing ?

Cheers,

Serge.
