google.cloud.firestore_v1.document.DocumentReference type missing from BaseCollectionReference

[jverkoey]

On this line:

https://github.com/googleapis/python-firestore/blob/bb482e99bd496489fb589a8fc154692585cef061/google/cloud/firestore_v1/base_collection.py#L131C1-L131C59

I'd expect it to have a return value of -> google.cloud.firestore_v1.document.DocumentReference. The absence of this return value is making it difficult to write type-safe code that makes use of document references.

[dconeybe]

Re-assigning to @daniel-sanche since I do not work in this repository.