docs: Update documentation.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/BigQueryDataTransfer/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:6018498e15310260dc9b03c9d576608908ed9fbabe42e1494ff3d827fea27b19
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/bigquery/datatransfer/artman_bigquerydatatransfer.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigquerydatatransfer-v1.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/DataTransferServiceGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/DataTransferServiceClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/DataTransferServiceClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V1/DataTransferServiceSmokeTest.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/StartManualTransferRunsRequest_TimeRange.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_Format.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/DataSourceParameter_Type.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ListTransferRunsRequest_RunAttempt.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_Encoding.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/DataSource_AuthorizationType.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/DataSource_DataRefreshType.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_RecordSchema.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_FieldSchema_Type.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_TableDefinition_CsvOptions.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_TableDefinition.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/ImportedDataInfo_FieldSchema.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/TransferMessage_MessageSeverity.php.
synthtool > No replacements made in src/V1/Gapic/DataTransferServiceGapicClient.php for pattern ListTransferRunsRequest_RunAttempt, maybe replacement is not longer needed?
synthtool > Replaced '@group datatransfer' in tests/Unit/V1/DataTransferServiceClientTest.php.
synthtool > Replaced '@group datatransfer' in tests/System/V1/DataTransferServiceSmokeTest.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #2040 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2040   +/-   ##
=========================================
  Coverage     92.56%   92.56%           
  Complexity     4374     4374           
=========================================
  Files           303      303           
  Lines         12996    12996           
=========================================
  Hits          12030    12030           
  Misses          966      966

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