Docs: Object Identification Spec Examples use non-global id values

The [Object Identification Spec docs](https://github.com/facebook/relay/blob/master/website/spec/ObjectIdentification.md#node-interface) describe `id` needing to be refetchable without additional type information and thus typical systems would need to encode some kind of type information into the `id` value itself. However, in the examples throughout the documentation, the `id` values used are simple small integers or stringified small integers. This has been causing us a lot of confusion as we are discussing our implementation. My proposed fix would be use realistic `id` values that encode the type information something like pseudo-code: `base64("user:" + "4")`. That way the doc examples would accurately reflect a compliant implementation and thus reduce confusion.