Document type limitations

[WesleyAC]

This is somewhat related to #36, but it would be great to know what kinds of types are and aren't supported. It seems like some more complex types fail in a way that's somewhat opaque, and it would be great to have docs to check what is and isn't supported.

[jerel]

Thanks for opening an issue. In addition to the alias type mentioned in 36 do you have another example of a type failing with a less-than-helpful error message? I'd like to fix any papercuts like that.

Off the top of my head there's things like using f32 or i32 which should fail (but they explain in the error message to use their 64 bit counterparts).