Add sentence about row ordering (googleapis#3504)

I'm not sure if this is the best place for this, but we want to make sure it's documented that rows are returned in row key order.

Author: garye

bigtable/google/cloud/bigtable/row_data.py

@@ -256,7 +256,7 @@ def consume_next(self):
         """Consume the next ``ReadRowsResponse`` from the stream.
 
         Parse the response and its chunks into a new/existing row in
-        :attr:`_rows`
+        :attr:`_rows`. Rows are returned in order by row key.
         """
         response = six.next(self._response_iterator)
         self._counter += 1