Spanner: 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/spanner/artman_spanner.yaml. synthtool > Ensuring dependencies. synthtool > Pulling artman image. synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/spanner-v1. synthtool > Replaced '# limitations under the License.\n\nimport google.api_core.grpc_helpers\n' in google/cloud/spanner_v1/gapic/transports/spanner_grpc_transport.py. synthtool > Replaced 'from google.cloud.spanner_v1.proto import spanner_pb2_grpc\n' in google/cloud/spanner_v1/gapic/transports/spanner_grpc_transport.py. synthtool > Replaced '(\\s+)return google.api_core.grpc_helpers.create_channel\\(\n' in google/cloud/spanner_v1/gapic/transports/spanner_grpc_transport.py. synthtool > Running generator for google/spanner/admin/instance/artman_spanner_admin_instance.yaml. synthtool > Ensuring dependencies. synthtool > Pulling artman image. synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/spanner-admin-instance-v1. synthtool > Replaced "'google-cloud-spanner-admin-instance'" in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py. synthtool > Replaced 'from google\\.cloud\\.spanner\\.admin\\.instance_v1.proto' in google/cloud/spanner_admin_instance_v1/proto/spanner_instance_admin_pb2_grpc.py. synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* The instance is readable via the API, with all requested attributes but no allocated resources. Its state is `CREATING`., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* Cancelling the operation renders the instance immediately unreadable via the API., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* Billing for all successfully-allocated resources begins \(some types may have lower than the requested levels\)., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* The instance and \*all of its databases\* immediately and irrevocably disappear from the API. All data in the databases is permanently deleted., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* ``labels.env:dev`` --> The instance has the label \\"env\\" and the value of :: the label contains the string \\"dev\\"., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* ``name:howl labels.env:dev`` --> The instance's name contains \\"howl\\" and :: it has the label \\"env\\" with its value containing \\"dev\\"., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* For resource types for which a decrease in the instance's allocation has been requested, billing is based on the newly-requested level., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* Cancelling the operation sets its metadata's \[cancel_time\]\[google.spanner.admin.instance.v1.UpdateInstanceMetadata.cancel_time\], and begins restoring resources to their pre-request values. The operation is guaranteed to succeed at undoing all resource changes, after which point it terminates with a `CANCELLED` status., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* Reading the instance via the API continues to give the pre-request resource levels., maybe replacement is not longer needed? synthtool > No replacements made in google/cloud/spanner_admin_instance_v1/gapic/instance_admin_client.py for pattern \* Billing begins for all successfully-allocated resources \(some types may have lower than the requested levels\). \* All newly-reserved resources are available for serving the instance's tables., maybe replacement is not longer needed? synthtool > Replaced '====*' in google/cloud/spanner_v1/proto/transaction_pb2.py. synthtool > Replaced '----*' in google/cloud/spanner_v1/proto/transaction_pb2.py. synthtool > Replaced '~~~~*' in google/cloud/spanner_v1/proto/transaction_pb2.py. synthtool > Running generator for google/spanner/admin/database/artman_spanner_admin_database.yaml. synthtool > Ensuring dependencies. synthtool > Pulling artman image. synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/spanner-admin-database-v1. synthtool > Replaced "'google-cloud-spanner-admin-database'" in google/cloud/spanner_admin_database_v1/gapic/database_admin_client.py. synthtool > Replaced 'from google\\.cloud\\.spanner\\.admin\\.database_v1.proto' in google/cloud/spanner_admin_database_v1/proto/spanner_database_admin_pb2_grpc.py. synthtool > No replacements made in google/cloud/spanner_admin_database_v1/gapic/database_admin_client.py for pattern database ID must be enclosed in backticks \(`` `` ``\)., maybe replacement is not longer needed? synthtool > Cleaned up 0 temporary directories. 

[tseaver]

Includes codegen fix for #5710.