Improve documentation of UnknownArrayHandle::IsType
Under some circumstances, `IsType` can return false for a type that exactly matches the type used to create the `UnknownArrayHandle`. This is because types like `ArrayHandleCast` get converted to the array they contain. Thus, `IsType` will return false when it is expected to be true. In most cases, you should use `CanConvert`. The `IsType` documentation now points you there.