[krbdev.mit.edu #6788] document preferred_preauth_types

[Sam Hartman via RT]

In a discussion on krbdev, Nico told Russ that krb5.conf could force
preauth.
Russ said he was unaware of that option; Nico thought he must have been
mistaken.
However, Quoting get_in_tkt.c:

    ret = krb5int_libdefault_string(context, realm,
    KRB5_CONF_PREFERRED_PREAUTH\
    _TYPES,
                                        &preauth_types);
                                        


This option should be documented.


--Sam