MAN: "user" doesn't work with socket-activated services
    
    Make it clear in the manual that the "user" option is not going to work
    with socket-activated services and also mention what's the best way to
    change it in case it's needed, being clear about what can go wrong in
    case the admin decides to do it.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2243
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>