docs(iam): mark required params as required in docstring (via synth)

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/iam/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:feed210b5723c6f524b52ef6d7740a030f2d1a8f7c29a71c5e5b4481ceaad7f5
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/iam/credentials/artman_iamcredentials_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iamcredentials-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/iam/credentials/v1/common.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iamcredentials-v1/google/cloud/iam_credentials_v1/proto/common.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/iam/credentials/v1/iamcredentials.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iamcredentials-v1/google/cloud/iam_credentials_v1/proto/iamcredentials.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iamcredentials-v1/google/cloud/iam_credentials_v1/proto.
synthtool > Replaced 'google-cloud-iamcredentials' in google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py.
synthtool > Replaced 'google-cloud-iamcredentials' in docs/conf.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/types.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/proto/iamcredentials_pb2.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/proto/iamcredentials_pb2_grpc.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in tests/unit/gapic/v1/test_iam_credentials_client_v1.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import iamcredentials_pb2_grpc' in google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import iamcredentials_pb2_grpc' in google/cloud/iam_credentials_v1/gapic/transports/iam_credentials_grpc_transport.py.
synthtool > Replaced '"""Attributes:\n' in google/cloud/iam_credentials_v1/proto/common_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/iam/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/gapic/iam_credentials_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/gapic/transports/iam_credentials_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/common_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/docs/conf.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/iamcredentials_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/iamcredentials_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/noxfile.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/tests/unit/gapic/v1/test_iam_credentials_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/common_pb2.py
All done! ✨ 🍰 ✨
15 files reformatted, 4 files left unchanged.
Session blacken was successful.
synthtool > Wrote metadata to synth.metadata.