title: Realtime update is returning only the changed document not the whole query
body: <p>I am using Firestore real time  update, I have the following listener</p>
  
  <pre><code> CollectionReference collecRef = db.collection("lists").document("doc1").collection("locations");
  changesListener=collecRef.addSnapshotListener(new EventListener&lt;QuerySnapshot&gt;() {
              @Override
              public void onEvent(@Nullable QuerySnapshot snapshots,
                                  @Nullable FirebaseFirestoreException e) {
              if (e != null) {
                  Log.w(TAG, "listen:error", e);
                  return;
              }
              a
              for (DocumentChange dc : snapshots.getDocumentChanges()) 
              {
                  DocumentSnapshot document = dc.getDocument();
               // this is where I expected to loop over all documents. But instead it is only one item..the changed the document
  
              }
  
          }
      });
  </code></pre>
  
  <p>According to docs:</p>
  
  <blockquote>
    <p>It is often useful to see the actual changes to query results between
    query snapshots, instead of simply using the entire query snapshot.</p>
  </blockquote>
  
  <p>This implies that when a document is modified then I am expecting the whole query to rerun since there is a change to the results. However, in the listener  I only get a documentsnapshot of the document that has been changed not all whole query results (ie all the documents in my case)</p>
  
  <p>Am I missing something here?</p>