Discuss cert expiry, no-key princs in PKINIT docs

In pkinit.rst, add "-days" options to the example commands for
creating certificate and briefly discuss the issue of expiration dates
so that the administrator thinks about it.  In troubleshoot.rst, add
an entry for the "certificate has expired" error which results from
PKINIT (when linked with OpenSSL) when a certificate has expired.

ticket: 7719 (new)
target_version: 1.12
tags: pullup