Reminder that we need to document `useFirestoreDocData` and `useFirestoreCollectionData` (implemented in #151). We should at least add them to the [API Reference](https://github.com/FirebaseExtended/reactfire/blob/master/docs/reference.md), and maybe add them to [Common Use Cases](https://github.com/FirebaseExtended/reactfire/blob/master/docs/use.md) too.