Add clarifying comment to blacken nox target.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/trace/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:5fd9aee1d82a00cebf425c8fa431f5457539562f5867ad9c54370f0ec9a7ccaa
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/devtools/cloudtrace/artman_cloudtrace_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/devtools/cloudtrace/v1/trace.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v1/google/cloud/trace_v1/proto/trace.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v1/google/cloud/trace_v1/proto.
synthtool > Replaced 'from google.devtools.cloudtrace_v1.proto import ' in google/cloud/trace_v1/proto/trace_pb2_grpc.py.
synthtool > Running generator for google/devtools/cloudtrace/artman_cloudtrace_v2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v2.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/devtools/cloudtrace/v2/trace.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v2/google/cloud/trace_v2/proto/trace.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/devtools/cloudtrace/v2/tracing.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v2/google/cloud/trace_v2/proto/tracing.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/trace-v2/google/cloud/trace_v2/proto.
synthtool > Replaced 'from google.devtools.cloudtrace_v2.proto import ' in google/cloud/trace_v2/proto/tracing_pb2_grpc.py.
synthtool > Replaced 'from google.devtools.cloudtrace_v2.proto import ' in google/cloud/trace_v2/proto/tracing_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/trace_v1/proto/trace_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/trace_v2/proto/trace_pb2.py.
synthtool > Replaced '(^.*$\\n)*' in google/cloud/trace_v2/proto/tracing_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/trace/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/gapic/trace_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/gapic/transports/trace_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/proto/trace_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/gapic/trace_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/gapic/transports/trace_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/gapic/trace_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/proto/trace_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/gapic/trace_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/proto/tracing_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/proto/tracing_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/tests/unit/gapic/v1/test_trace_service_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/tests/unit/gapic/v2/test_trace_service_client_v2.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v1/proto/trace_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/trace/google/cloud/trace_v2/proto/trace_pb2.py
All done! ✨ 🍰 ✨
20 files reformatted, 23 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.