Asset: fix client_info bug, update docstrings.

[dpebot]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/asset/artman_cloudasset_v1beta1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/asset-v1beta1.
synthtool > Replaced 'from google.iam.v1 import policy_pb2 as' in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Replaced 'from google.iam.v1 import iam_policy_pb2_grpc as google_dot_iam_dot_v1_dot_policy__pb2' in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Replaced "_ASSET.fields_by_name\\['iam_policy'\\].message_type = google_dot_iam_dot_v1_dot_policy__pb2._POLICY" in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Replaced '          The full name of the asset. For example: ``//compute.googleapi\n          s.com/projects/my_project_123/zones/zone1/instances/instance1`\n          `. See `Resource Names <https://cloud.google.com/apis/design/r\n          esource_names#full_resource_name>`__ for more information.\n' in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Cleaned up 0 temporary directories.

[tseaver]

Includes codegen fix for #5710.