chore: add note about preference of stream() to get() (#3972)

* Add note about CollectionRef get deprecation.

* Update note.

Co-authored-by: gcf-merge-on-green[bot] <60162190+gcf-merge-on-green[bot]@users.noreply.github.com>
Co-authored-by: Takashi Matsuo <tmatsuo@google.com>

firestore/cloud-client/snippets.py

@@ -244,6 +244,7 @@ def get_custom_class():
 def get_simple_query():
     db = firestore.Client()
     # [START get_simple_query]
+    # Note: Use of CollectionRef stream() is prefered to get()
     docs = db.collection(u'cities').where(u'capital', u'==', True).stream()
 
     for doc in docs: