Update comments for RFC 3244 kpasswd extensions

Remove the inaccurate comments "unused" and replace them with
more useful representations of what they mean.

(cherry picked from commit f2cec6d908a8b15908f4de199540e5ad3486882b)

https://github.com/krb5/krb5/commit/1b6c19663412c03917ffb92d78beb98ca4b1a10c
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 1b6c19663412c03917ffb92d78beb98ca4b1a10c
Branch: krb5-1.11
 src/include/krb5/krb5.hin |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)