sudo: document background activity
    
    When we introduced socket activation, we changed the internall behaviour.
    Previously we disabled sudo if it was not listed in services, with
    socket activation we removed this feature. Some users were confused
    so this change documents current behaviour.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>