Add UI location for configuring valid goto URLs

Peter pointed out on the FR forums here:

https://forum.forgerock.com/topic/how-to-redirect-after-login-2/#post-18360

The following section doesn't mention where to configure the valid goto URLs:

https://backstage.forgerock.com/docs/am/5.1/authentication-guide/#configure-valid-goto-url-resources

We should add a note to say where to go to configure the property.