Document that newer AFS supports stronger crypto

Modern OpenAFS releases support using encryption stronger than single
DES with Kerberos.  Update the documentation accordingly.

(cherry picked from commit 9b51ffb0c55a6c4c44501d86eb207acc79403c5c)

ticket: 7761
version_fixed: 1.13
status: resolved
Better document how to verify PGP signature

Add text clarifying our unusual packaging of the PGP signature inside
a tar file.

(cherry picked from commit fa4138c7853487105ab3c54e6d176c45eaf8b065)

ticket: 7927
version_fixed: 1.13
status: resolved
Update mitK5features.rst for 1.13
Updates for krb5-1.13
Update for krb5-1.13-postrelease