Minor improvements to the REST principles chapter

Well written summary!
While reading through, I made some clarifications (from my POV).

In addendum, see [a post of mine clarifying REST concepts](http://hyperrest.github.io/2013-03-30-rest-api-concepts/#content). To a great extent, I think it still is a good analogy (1.5y after).
Well Fielding says "By separating the user interface concerns from the data storage concerns..." so I just expanded on that. Not sure what lead you to Riak.

Also the server does not necessarily care about linking the content together at all, broken links are common.