Add more descriptions in the docs

mmv1/third_party/terraform/website/docs/r/bigquery_dataset_iam.html.markdown

@@ -90,7 +90,7 @@ The following arguments are supported:
   * **allUsers**: A special identifier that represents anyone who is on the internet; with or without a Google account.
   * **domain:{domain}**: A G Suite domain (primary, instead of alias) name that represents all the users of that domain. For example, google.com or example.com.
   * **group:{emailid}**: An email address that represents a Google group. For example, [email protected].
-  * **iamMember:{principal}**: Some other type of member that appears in the IAM Policy but isn't a user, group, domain, or special group.
+  * **iamMember:{principal}**: Some other type of member that appears in the IAM Policy but isn't a user, group, domain, or special group. This is used for example for workload/workforce federated identities (principal, principalSet).
   * **serviceAccount:{emailid}**: An email address that represents a service account. For example, [email protected].
   * **user:{emailid}**: An email address that represents a specific Google account. For example, [email protected] or [email protected].