Document API for getting anonymous tickets

In init_creds.rst, document how to get anonymous credentials from an
application.

(cherry picked from commit 9973812776fc69bc5fb8e3c7f85784cb295625f0)

https://github.com/krb5/krb5/commit/d4f2c5ce0c54400ca3ade0af55e5b636fdbe9f40
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: d4f2c5ce0c54400ca3ade0af55e5b636fdbe9f40
Branch: krb5-1.11
 doc/admin/pkinit.rst      |    2 ++
 doc/appdev/init_creds.rst |   26 ++++++++++++++++++++++++++
 2 files changed, 28 insertions(+), 0 deletions(-)