add more types to `google.cloud.firestore.BaseDocumentReference`

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this feature request.

### Summary of the feature request

many methods of `{Async,}DocumentReference` (such as `id`, `path`, `collections`, …) are missing some type information, making my strict type checker complain about using `Any` (type used when not documented). having theses types in the codebase directly will help ensure that we are correctly using theses references.

### Desired code experience

```python
_: int = reference.path
```

### Expected results

lint fails with "Incompatible types in assignment"

### API client name and version

_No response_

### Use case

catch errors sooner

### Additional context

_No response_