Document krb5_kt_next_entry() requirement

Successful calls to krb5_kt_next_entry() return a krb5_keytab_entry
that the caller is responsible for freeing.  Note this, and the
proper function to do so, in the doxygen comments.

(cherry picked from commit e70c73b804400a2f18a044dd386dbf1a8430b92e)

https://github.com/krb5/krb5/commit/3cac667399ebce50985242f792526455ce59ae0d
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 3cac667399ebce50985242f792526455ce59ae0d
Branch: krb5-1.14
 src/include/krb5/krb5.hin |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)