Update self registration.

Documentation/UseCase.rst

@@ -58,7 +58,7 @@ Concept:
     * Only send mail if the mail address is known!
     * If email is not found, the user should not be noticed.
     * Create a new ``Person.auth`` token.
-    * Set ``Person.authExpired = NOW + ? days``.
+    * Set ``Person.authExpired = NOW() + INTERVAL 4 DAY``.
 
 * When the user opens the URL, the form 'Set password' is shown.