chore: add note about preference of stream() to get()

[sdlin]

This snippet is used in these docs:
https://cloud.google.com/firestore/docs/query-data/queries#python_3

Which say:

After creating a query object, use the get() function to retrieve the results:

That's true for the other languages, but not for Python, which has deprecated get() and replaced it with stream(), based on these docs:
https://googleapis.dev/python/firestore/latest/collection.html#google.cloud.firestore_v1.collection.CollectionReference.get

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