docs: add warning about unavailable fields; update templates (via synth)

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

2020-04-21 06:08:34,971 synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
On branch autosynth
nothing to commit, working tree clean
2020-04-21 06:08:35,039 synthtool > Ensuring dependencies.
2020-04-21 06:08:35,044 synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:b3b47805231a305d0f40c4bf069df20f6a2635574e6d4259fac651d3f9f6e098
Status: Image is up to date for googleapis/artman:latest
2020-04-21 06:08:36,678 synthtool > Cloning googleapis.
2020-04-21 06:08:37,333 synthtool > Running generator for google/cloud/billing/budgets/artman_billingbudgets_v1beta1.yaml.
2020-04-21 06:08:52,256 synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1.
2020-04-21 06:08:52,258 synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/billing/budgets/v1beta1/budget_service.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1/google/cloud/billingbudgets_v1beta1/proto/budget_service.proto
2020-04-21 06:08:52,258 synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/billing/budgets/v1beta1/budget_model.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1/google/cloud/billingbudgets_v1beta1/proto/budget_model.proto
2020-04-21 06:08:52,258 synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/billingbudgets-v1beta1/google/cloud/billingbudgets_v1beta1/proto.
2020-04-21 06:08:52,279 synthtool > Replaced 'from google\\.cloud\\.billing\\.budgets_v1beta1' in google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2.py.
2020-04-21 06:08:52,279 synthtool > Replaced 'from google\\.cloud\\.billing\\.budgets_v1beta1' in google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2_grpc.py.
2020-04-21 06:08:52,284 synthtool > Replaced 'google-cloud-billingbudgets' in google/cloud/billing_budgets_v1beta1/gapic/budget_service_client.py.
2020-04-21 06:08:52,286 synthtool > Replaced 'google-cloud-billingbudgets' in docs/conf.py.
.coveragerc
.flake8
.github/CONTRIBUTING.md
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.github/PULL_REQUEST_TEMPLATE.md
.github/release-please.yml
.gitignore
.kokoro/build.sh
.kokoro/continuous/common.cfg
.kokoro/continuous/continuous.cfg
.kokoro/docs/common.cfg
.kokoro/docs/docs.cfg
.kokoro/presubmit/common.cfg
.kokoro/presubmit/presubmit.cfg
.kokoro/publish-docs.sh
.kokoro/release.sh
.kokoro/release/common.cfg
.kokoro/release/release.cfg
.kokoro/trampoline.sh
CODE_OF_CONDUCT.md
CONTRIBUTING.rst
LICENSE
MANIFEST.in
docs/_static/custom.css
docs/_templates/layout.html
docs/conf.py.j2
noxfile.py.j2
renovate.json
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/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/gapic/budget_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/docs/conf.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/proto/budget_model_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/gapic/transports/budget_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/gapic/budget_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/noxfile.py
reformatted /tmpfs/src/git/autosynth/working_repo/tests/unit/gapic/v1beta1/test_budget_service_client_v1beta1.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/proto/budget_service_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/google/cloud/billing_budgets_v1beta1/proto/budget_model_pb2.py
All done! ✨ 🍰 ✨
16 files reformatted, 4 files left unchanged.
Session blacken was successful.
2020-04-21 06:08:55,824 synthtool > Wrote metadata to synth.metadata.