Datastore: 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/datastore/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/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 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. 

[busunkim96]

Ignoring failing tests in unrelated APIs: BigQuery, Logging, Phishing Protection, Spanner, Storage, reCAPTCHA Enterprise