Document prompter and responder callbacks

(cherry picked from commit 08aeb0cbc6e72e14c0ec3424363539e61507c4e8)

https://github.com/krb5/krb5/commit/745a3f3847dac60f3e2d01f486969b660b692927
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 745a3f3847dac60f3e2d01f486969b660b692927
Branch: krb5-1.11
 doc/appldev/init_creds.rst |   85 ++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 85 insertions(+), 0 deletions(-)