Update documentation (#6859)

Update documentation to reflect the correct format of user_id (fully qualified).

docs/admin_api/user_admin_api.rst

@@ -2,7 +2,8 @@ Create or modify Account
 ========================
 
 This API allows an administrator to create or modify a user account with a
-specific ``user_id``.
+specific ``user_id``. Be aware that ``user_id`` is fully qualified: for example,
+``@user:server.com``.
 
 This api is::