library-level docs for collections

Adds a high-level discussion of "what collection should you use for what", as well as some general discussion of correct/efficient usage of the capacity, iterator, and entry APIs.

Still building docs to confirm this renders right and the examples are good, but the content can be reviewed now.
But a `Vec` is still just one value. The fact that you can use it to *encode* a multimap doesn't change the basic semantics of a map.