Pub/Sub: wrap subscriber in a with block and add comments (#4070)
Pub/Sub: wrap subscriber in a with block and add comments (#4070)

Use a `with` block to wrap subscriber and describe its purpose. 

Internal bug: b/157401623
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples