Unify id requirement level across spec

The `id` requirement-level was inconsistent between the ID-based and URL-based formats. Based on a [GitHub discussion][ghd], it seems like `id` keys are *usually* a good thing, but there are cases where a hard requirement would be unnecessary.

I have also added a [quick blurb][qb] that notes the possible omission, but I think it might require some additional explanation.

[ghd]: json-api#181
[qb]: https://github.com/andrewsardone/json-api/commit/51ee09f68740bf47635f8a7cacc6c3902b18a637

Closes json-api/json-api#181.