Pub/Sub: Add nox session docs (via synth).

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/pubsub/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:c58f4ec3838eb4e0718eb1bccc6512bd6850feaa85a360a9e38f6f848ec73bc2
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/pubsub/artman_pubsub.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/pubsub-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/pubsub/v1/pubsub.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/pubsub-v1/google/cloud/pubsub_v1/proto/pubsub.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/pubsub-v1/google/cloud/pubsub_v1/proto.
synthtool > Replaced 'from google.cloud import pubsub_v1' in tests/unit/gapic/v1/test_publisher_client_v1.py.
synthtool > Replaced ' pubsub_v1' in tests/unit/gapic/v1/test_publisher_client_v1.py.
synthtool > Replaced 'from google.cloud import pubsub_v1' in tests/unit/gapic/v1/test_subscriber_client_v1.py.
synthtool > Replaced ' pubsub_v1' in tests/unit/gapic/v1/test_subscriber_client_v1.py.
synthtool > Replaced '# The name of the interface for this client. This is the key used to' in google/cloud/pubsub_v1/gapic/subscriber_client.py.
synthtool > Replaced '# The name of the interface for this client. This is the key used to' in google/cloud/pubsub_v1/gapic/publisher_client.py.
synthtool > Replaced 'import google.api_core.gapic_v1.method\n' in google/cloud/pubsub_v1/gapic/publisher_client.py.
synthtool > Replaced 'DESCRIPTOR = _MESSAGESTORAGEPOLICY,\n\\s+__module__.*\n\\s+,\n\\s+__doc__ = """' in google/cloud/pubsub_v1/proto/pubsub_pb2.py.
synthtool > No replacements made in google/cloud/pubsub_v1/gapic/subscriber_client.py for pattern subscription \(str\): The subscription whose backlog .*
(.*
)+?\s+Format is .*, maybe replacement is not longer needed?
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/pubsub/.nox/blacken
pip install black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/gapic/publisher_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/gapic/subscriber_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/gapic/transports/publisher_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/gapic/transports/subscriber_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/gapic/publisher_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/gapic/subscriber_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/proto/pubsub_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/tests/system/gapic/v1/test_system_publisher_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/tests/unit/gapic/v1/test_publisher_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/tests/unit/gapic/v1/test_subscriber_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/pubsub/google/cloud/pubsub_v1/proto/pubsub_pb2.py
All done! ✨ 🍰 ✨
14 files reformatted, 45 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 3 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[busunkim96]

Ignoring failing tests in unrelated APIs: Bigtable, Phishing Protection, Spanner,reCAPTCHA Enterprise