docs: [bigquery-datatransfer] add a note to the CreateTransferConfigR…

…equest and UpdateTransferConfigRequest to disable restricting service account usage (#5657)

* docs: add a note to the CreateTransferConfigRequest and UpdateTransferConfigRequest to disable restricting service account usage

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packages/google-cloud-bigquery-datatransfer/protos/google/cloud/bigquery/datatransfer/v1/datatransfer.proto

@@ -496,6 +496,11 @@ message ListDataSourcesResponse {
 // associated with the user id corresponding to the authorization info.
 // Otherwise, the transfer configuration will be associated with the calling
 // user.
+//
+// When using a cross project service account for creating a transfer config,
+// you must enable cross project service account usage. For more information,
+// see [Disable attachment of service accounts to resources in other
+// projects](https://cloud.google.com/resource-manager/docs/organization-policy/restricting-service-accounts#disable_cross_project_service_accounts).
 message CreateTransferConfigRequest {
   // Required. The BigQuery project id where the transfer configuration should
   // be created. Must be in the format
@@ -563,6 +568,11 @@ message CreateTransferConfigRequest {
 
 // A request to update a transfer configuration. To update the user id of the
 // transfer configuration, authorization info needs to be provided.
+//
+// When using a cross project service account for updating a transfer config,
+// you must enable cross project service account usage. For more information,
+// see [Disable attachment of service accounts to resources in other
+// projects](https://cloud.google.com/resource-manager/docs/organization-policy/restricting-service-accounts#disable_cross_project_service_accounts).
 message UpdateTransferConfigRequest {
   // Required. Data transfer configuration to create.
   TransferConfig transfer_config = 1 [(google.api.field_behavior) = REQUIRED];