Add doxygen comments for RFC 8009, RFC 4757

The aes-sha2 specification has been published as RFC 8009.  Add
Doxygen comments to the #defines for its enctype and checksum type
comments.  Also add comments for the RC4 enctype and checksum type
constants referring to RFC 4757.

(cherry picked from commit bd895523179e7f639c40527ccd0af6d22cbbf364)

https://github.com/krb5/krb5/commit/bceaca97c75c5477b3e57cd7588c455a7b47c51a
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: bceaca97c75c5477b3e57cd7588c455a7b47c51a
Branch: krb5-1.15
 src/include/krb5/krb5.hin |   16 ++++++++--------
 1 files changed, 8 insertions(+), 8 deletions(-)