Make it (even more) obvious you can supply any appropriate variable to add_host

I managed to miss the sentence "Takes variables so you can define the new hosts more fully" when reading the docs for `add_host`.

This is pretty important functionality and I think it would be good if it was spelled out in the "glance-able" Options section.

I realise the examples also show this in practice but from looking at the Options section alone I think it should be possible to tell there can be more options supplied than "groups and name".