Suggestion for improvements to x509.pod

This commit is a suggestion to hopefully improve x509.pod. I had to
re-read it the first time through and with these changes it reads a
little easier, and wondering if others agree.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/4924)