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?
When collecting something, it's generally better to put the type annotation on the collect rather than the variable. But it might look gross here with the `<<>>`. Hmm