Update translator documentation.

lib/designsettings.php

@@ -119,6 +119,7 @@ class DesignSettingsAction extends SettingsAction
         // CSRF protection
         $token = $this->trimmed('token');
         if (!$token || $token != common_session_token()) {
+            // TRANS: Client error displayed when the session token does not match or is not given.
             $this->showForm(_('There was a problem with your session token. '.
                               'Try again, please.'));
             return;