docs(documentai): add 2.7 deprecation warning (via synth)

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/documentai/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:264654a37596a44b0668b8ce6ac41082d713f6ee150b3fc6425fa78cc64e4f20
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/documentai/artman_documentai_v1beta1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/documentai-v1beta1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/documentai/v1beta1/document_understanding.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/documentai-v1beta1/google/cloud/documentai_v1beta1/proto/document_understanding.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/documentai/v1beta1/document.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/documentai-v1beta1/google/cloud/documentai_v1beta1/proto/document.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/documentai/v1beta1/geometry.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/documentai-v1beta1/google/cloud/documentai_v1beta1/proto/geometry.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/documentai-v1beta1/google/cloud/documentai_v1beta1/proto.
synthtool > Replaced '\\| Specifies a known document type for deeper structure\n          detection\\. Valid   values are currently "general" and\n          "invoice"\\. If not provided,   "general" \\| is used as default.\n          If any other value is given, the request is   rejected\\.' in google/cloud/documentai_v1beta1/proto/document_understanding_pb2.py.
.coveragerc
.flake8
MANIFEST.in
docs/_static/custom.css
docs/_templates/layout.html
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtual environment (virtualenv) using python3.6 in .nox/blacken
pip install black==19.3b0
black docs google tests noxfile.py setup.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/gapic/document_understanding_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/gapic/transports/document_understanding_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/proto/document_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/docs/conf.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/proto/document_understanding_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/gapic/document_understanding_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/proto/geometry_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/noxfile.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/proto/geometry_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/tests/unit/gapic/v1beta1/test_document_understanding_service_client_v1beta1.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/proto/document_understanding_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/documentai/google/cloud/documentai_v1beta1/proto/document_pb2.py
All done! ✨ 🍰 ✨
18 files reformatted, 4 files left unchanged.
Session blacken was successful.
synthtool > Wrote metadata to synth.metadata.