flatten declaration typings

The TypeScript declarations for flatten say:

```
Note: `flatten(true)` operates on Collection<any, Collection<K, V>> and
returns Collection<K, V>
```

Would it make sense to add another typing like this:

```ts
flatten(true): V;
```

or maybe some additional generics on it like this:

```ts
flatten<K2 = any, V2  = any>(shallow?: boolean): Collection<K2, V2>;
```