Multiple stream example needs clarify

When use 1 stream following this example https://github.com/googleapis/python-bigquery-storage/blob/6254bf2a588e69e2175df1c67edb514655d93e9d/samples/to_dataframe/main_test.py#L130-L139, the example perhaps make confusion.

The example states that we can read from multiple streams in order to get data faster. It does not say that multiple streams may be generated automatically if there is a lot of data and so you need to get data from all streams.

Thanks!