PubSub: Document how to choose the PubSub auth method (#8429)
PubSub: Document how to choose the PubSub auth method (#8429)

* Document how to choose the PubSub auth method

* Give more exposure to google-auth in PubSub docs

The link to the library is moved to the first paragraph of the
Authentication section for better visibility.

* Use publisher audience for the publisher client

GoogleCloudPlatform/gcloud-python,tswast/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,googleapis/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python