Clarify the role of id keys within the URL-based JSON API

This comes off of a discussion in json-api/json-api#179 and @steveklabnik's [call for a PR][cpr].

I'm not sure if the additions align with what the team had in mind, but I wrote down [my earlier thoughts][met] that are based on some experiences implementing a URL-based JSON API.

[cpr]: https://github.com/json-api/json-api/pull/179#issuecomment-32909054
[met]: https://github.com/json-api/json-api/pull/179#issuecomment-32897319