Add GSS credential store documentation

Add documentation for gss_acquire_cred_from() and
gss_store_cred_into(), including descriptions of the currently
supported options for the krb5 mechanism.

https://github.com/krb5/krb5/commit/d82de27053ae8110b65e71480db8f8504ae53903
Author: Greg Hudson <ghudson@mit.edu>
Commit: d82de27053ae8110b65e71480db8f8504ae53903
Branch: master
 doc/appdev/gssapi.rst |   80 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 80 insertions(+), 0 deletions(-)