Wrap 'URL-Based JSON API > IDs' section to 80 chars

This appears to be the convention that's being used more and more in
sections of the document.
Clarify the role of URLs being used as IDs

Per a [discussion][1] on the best practice of the `id` key within the
URL-based format.

[1]: https://github.com/json-api/json-api/pull/179
Merge pull request #180 from andrewsardone/id-and-urls

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