Securitycenter: protoc-generated serialization update, docstring tweak.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/securitycenter/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:5a96c2c5c6f9570cc9556b63dc9ce1838777fd9166b5b64e43ad8e0ecee2fe2c
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/securitycenter/artman_securitycenter_v1beta1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/securitycenter-v1beta1.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/securitycenter_service_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/asset_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/organization_settings_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/finding_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/security_marks_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/securitycenter_v1beta1/proto/source_pb2.py.
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/securitycenter/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/gapic/transports/security_center_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/asset_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/gapic/security_center_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/finding_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/organization_settings_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/organization_settings_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/finding_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/asset_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/security_marks_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/security_marks_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/source_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/source_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/gapic/security_center_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/securitycenter_service_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/tests/unit/gapic/v1beta1/test_security_center_client_v1beta1.py
reformatted /tmpfs/src/git/autosynth/working_repo/securitycenter/google/cloud/securitycenter_v1beta1/proto/securitycenter_service_pb2.py
All done! ✨ 🍰 ✨
22 files reformatted, 4 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[tseaver]

Minor .proto file changes (comments only).