Document key usage assigned number conflict

Document key usage assigned number conflict

Document the fact that the key usage type 26 is used by both
KBKRB5_KEYUSAGE_PA_S4U_X509_USER_REQUEST and
KRB5_KEYUSAGE_PA_S4U_X509_USER_REQUEST, while 27 - by
KRB5_KEYUSAGE_PA_S4U_X509_USER_REPLY and KRB5_KEYUSAGE_PA_SAM_RESPONSE.

Also, since KRB5_KEYUSAGE_PA_REFERRAL is not actually used in MIT Kerberos
code and is not defined in the latest referrals draft
(http://tools.ietf.org/html/draft-ietf-krb-wg-kerberos-referrals-15)
mark it as "unused".

ticket: 7474
tags: pullup
target_version: 1.11