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/datastore/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/datastore/artman_datastore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/datastore-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/datastore/v1/entity.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/datastore-v1/google/cloud/datastore_v1/proto/entity.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/datastore/v1/datastore.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/datastore-v1/google/cloud/datastore_v1/proto/datastore.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/datastore/v1/query.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/datastore-v1/google/cloud/datastore_v1/proto/query.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/datastore-v1/google/cloud/datastore_v1/proto.
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/datastore/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/gapic/datastore_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/gapic/transports/datastore_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/proto/datastore_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/proto/entity_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/gapic/datastore_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/proto/query_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/proto/entity_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/proto/query_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/datastore/google/cloud/datastore_v1/proto/datastore_pb2.py
All done! ✨ 🍰 ✨
10 files reformatted, 37 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.