Clarify krb5_kt_resolve() API documentation

Explicitly say to use krb5_kt_close() like we do for most other
allocating API calls.  Note the default type.  Instead of saying "The
key table is not opened," say that the keytab file for FILE keytabs is
not opened by this call.

https://github.com/krb5/krb5/commit/5ab2dc42b3ae8e07f5cf7eb91b43e4f2ad338728
Author: Greg Hudson <ghudson@mit.edu>
Commit: 5ab2dc42b3ae8e07f5cf7eb91b43e4f2ad338728
Branch: master
 src/include/krb5/krb5.hin |   11 ++++++-----
 1 files changed, 6 insertions(+), 5 deletions(-)