Document trie collections

I'm a bit unsure over the map instantiations

```
let map: TrieMap<&str> = [(1, "a"), (2, "b"), (3, "c")].iter().map(|&x| x).collect();
```

Seems a bit long and ugly, but maybe it's fine?
Could just merge this into the previous sentence "... as a radix tree optimized for unsigned integer keys", or remove it entirely (since it *only* works with unsigned integer keys).