Fix #423:  Expose the 'more_results' value fetched from the back-end.

Include notes that it is known to be currently un-useful, since the
back-end sets it to indicate there are more results willy-nilly.

Fixes #423.
Prefer to use the value from `datastore_v1_pb2`:
```
datastore_v1_pb2.QueryResultBatch.MORE_RESULTS_AFTER_LIMIT
```