Document secure cookie format and callbacks

In kdcpreauth.rst, describe the set_cookie and get_cookie callbacks
and explain how to generate a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error
for multi-round-trip mechanisms.  Add a new file formats/cookie.rst
documenting the secure cookie format.

https://github.com/krb5/krb5/commit/752574e288c8322001d1b4e62f5fd99c579a3403
Author: Greg Hudson <ghudson@mit.edu>
Commit: 752574e288c8322001d1b4e62f5fd99c579a3403
Branch: master
 doc/formats/cookie.rst       |   60 ++++++++++++++++++++++++++++++++++++++++++
 doc/formats/index.rst        |    1 +
 doc/plugindev/kdcpreauth.rst |   14 ++++++++++
 3 files changed, 75 insertions(+), 0 deletions(-)