[krbdev.mit.edu #7474] Update comments about conflicting KRB5_KEYUSAGE_PA types

[Zhanna Tsitkova via RT]

Be more specific about the assignment-number conflict between the
KRB5_KEYUSAGE_PA types S4U_X509_USER_REQUEST/S4U_X509_USER_REPLY on one
side and SAM_CHALLENGE_TRACKID/SAM_RESPONSE on the other side. 
Otherwise, Doxygen comments look uninformative.