docs: Fix `run_in_transaction` return value docs. (#9264)
docs: Fix `run_in_transaction` return value docs. (#9264)


tswast/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,tswast/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,GoogleCloudPlatform/gcloud-python,GoogleCloudPlatform/gcloud-python