Update Non-interactive session refresh.md

Signed-off-by: Jan-Otto Kröpke <mail@jkroepke.de>

docs/Non-interactive session refresh.md

@@ -6,7 +6,8 @@ This means users must log in interactively each time they authenticate, includin
 
 However, you can change this behavior by enabling the `oauth2.refresh.enabled=true` setting.
 This allows `openvpn-auth-oauth2` to store either the connection ID or SessionID (`oauth2.refresh.use-session-id=true`),
-accepting connections without additional login checks.
+accepting connections without additional login checks. SessionIDs are availible in OpenVPN, if
+`auth-gen-token [lifetime] external-auth` is configured on server side.
 
 When `oauth2.refresh.validate-user=true` is set, `openvpn-auth-oauth2`
 requests a [refresh token](https://auth0.com/blog/refresh-tokens-what-are-they-and-when-to-use-them/)