Fix link to Json Value type in interop guide

[jvoigtlaender]

Part of fixing #450.

[evancz]

I think it'd be good to make it clear somehow that Json.Decode.Value can go through too. I couldn't figure out a module structure that made this pleasant. Maybe we can do it in words though. Like:

JavaScript values like Json.Encode.Value and Json.Decode.Value, allowing arbitrary things through in a safe way.

[jvoigtlaender]

Yeah, pointing out both Json.Encode.Value and Json.Decode.Value would be good. It would also be good, though, to make clear that both are actually the same type. Currently, the documentation at http://package.elm-lang.org/packages/elm-lang/core/3.0.0/Json-Decode#Value reads thus:

type alias Value = Value

It would be so much better if it instead were to read thus:

type alias Value = Json.Encode.Value

That is, to have this feature back.

[evancz]

Sure, I saw your comment about that, but it's not blocking anything in this issue.