Generate a getMore command document.

def _gen_get_more_command(cursor_id, coll, batch_size, max_await_time_ms):
    """Generate a getMore command document."""
    cmd = SON([('getMore', cursor_id),
               ('collection', coll)])
    if batch_size:
        cmd['batchSize'] = batch_size
    if max_await_time_ms is not None:
        cmd['maxTimeMS'] = max_await_time_ms
    return cmd