diff --git a/doc/usage.mdwn b/doc/usage.mdwn
index 4af74ecec..136e969c2 100644
--- a/doc/usage.mdwn
+++ b/doc/usage.mdwn
@@ -220,6 +220,8 @@ configuration options of their own.
   and banning of users; other powers may be added later. May be specified
   multiple times for multiple admins.
 
+  For an openid user specify the full URL of the login, including "http://".
+
 * --plugin name
 
   Enables the use of the specified [[plugin|plugins]] in the wiki.