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)

https://github.com/krb5/krb5/commit/0d551dad36c5d91a93f1a8dd70bee25fb10c42aa
Author: Tom Yu <tlyu@mit.edu>
Commit: 0d551dad36c5d91a93f1a8dd70bee25fb10c42aa
Branch: krb5-1.13
 doc/build/index.rst |   24 ++++++++++++++----------
 1 files changed, 14 insertions(+), 10 deletions(-)