Tighten up documentation on NUMBER and DATETIME in the guide

[Pike]

In fuent.js, we restricted the options to NUMBER and DATETIME beyond what's documented in the guide right now.

We should port those changes over.

[stasm]

That was going to be my next task after projectfluent/fluent.js#469 :) I think I'd like to completely remove the docs on which options are supported for now. As it stands, we can't really guarantee all options are supported across implementations.