diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index e428592..524c0fa 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -139,6 +139,11 @@
  #. ``alt_password``
  #. ``alt_tenant_name``
 
+If using Identity API v3, use the ``domain_name`` option to specify a
+domain other than the default domain.  The ``auth_version`` setting is
+used to switch between v2 (``v2``) or v3 (``v3``) versions of the Identity
+API.
+
 And in the ``auth`` section:
 
  #. ``use_dynamic_credentials = False``