Add GSSAPI S4U documentation

Describe how a GSS application can perform S4U2Self and S4U2Proxy
requests using the MIT krb5 GSS library.  Also add a reference to RFC
7546 at the top, and fix a reference to gssapi_krb5.h.

(cherry picked from commit 31fcadd6bef5e3fbcc986220b860a1af8c7030a1)

https://github.com/krb5/krb5/commit/a34dd07a5fe76e69d6265af346d70e6e8b385652
Author: Greg Hudson <ghudson@mit.edu>
Commit: a34dd07a5fe76e69d6265af346d70e6e8b385652
Branch: krb5-1.15
 doc/appdev/gssapi.rst |   84 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 files changed, 79 insertions(+), 5 deletions(-)