MAN: libkrb5 and SSSD use different expansions
    
    Users often wrongly use SSSD expansions in libkrb5 expansion template
    for principals. State explicitly it won't work.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2528
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 33b8bf140b1d82d2626eeeaaea29af49dcdb3c99)