Document preauth flags for service principals

These flags are overloaded to mean different things for clients and
servers; previously we only documented the client behavior.

(cherry picked from commit 7425e9b69566c241c54eb2686fb37f216122423f)

https://github.com/krb5/krb5/commit/a1faf888a98ea215c6a8589769f43ed024c6597f
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: a1faf888a98ea215c6a8589769f43ed024c6597f
Branch: krb5-1.11
 doc/admin/admin_commands/kadmin_local.rst |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)