Update IAM template docs to cover WIF principals (#15131)

Co-authored-by: Riley Karson <[email protected]>

Author: cherriford

mmv1/templates/terraform/resource_iam.html.markdown.tmpl

@@ -230,6 +230,7 @@ The following arguments are supported:
   * **projectOwner:projectid**: Owners of the given project. For example, "projectOwner:my-example-project"
   * **projectEditor:projectid**: Editors of the given project. For example, "projectEditor:my-example-project"
   * **projectViewer:projectid**: Viewers of the given project. For example, "projectViewer:my-example-project"
+  * **Federated identities**: One or more federated identities in a workload or workforce identity pool, workload running on GKE, etc. Refer to the [Principal identifiers documentation](https://cloud.google.com/iam/docs/principal-identifiers#allow) for examples of targets and valid configuration. For example, "principal://iam.googleapis.com/locations/global/workforcePools/example-contractors/subject/[email protected]"
 
 * `role` - (Required) The role that should be applied. Only one
     `{{ $.IamTerraformName }}_binding` can be used per role. Note that custom roles must be of the format