Add docs for 'result_index' usage and a system test.

[daspecster]

Closes #3166.

Updated the docs and added a system test for using result_index.

[lukesneeringer]

No code change required?

[daspecster]

@lukesneeringer turns out there isn't!
Since streaming is just returning the proto for now.

[dhermes]

It seems that is not correct: https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/1455

(Pdb) results
<google.cloud.speech.result.StreamingSpeechResult object at 0x7f20454e6a90>
(Pdb) results.__dict__
{'alternatives': [<google.cloud.speech.alternative.Alternative object at 0x7f20454e6ac8>], 'is_final': True, 'stability': 0.0}
(Pdb) results.alternatives
[<google.cloud.speech.alternative.Alternative object at 0x7f20454e6ac8>]
(Pdb) results.alternatives[0]
<google.cloud.speech.alternative.Alternative object at 0x7f20454e6ac8>
(Pdb) results.alternatives[0].__dict__
{'_transcript': 'hello thank you for using Google Cloud platform', '_confidence': 0.9826789498329163}