Define "link" objects in Relationships section

Per @dgeb's [suggestion][s].

[s]: https://github.com/json-api/json-api/pull/187#issuecomment-33905900
Remove code-literal wrapping of "link" objects

Per @dgeb's [comment][c], these objects won't appear in the JSON under a
literal `"link"` key, so they shouldn't written as such.

[c]: https://github.com/json-api/json-api/pull/187#issuecomment-33905900