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.