Improves certificates HOWTO

* adds links to various related documents.
* fixes a few typos.
* rewords a few sentences.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit 67472bd82bed9d5e481b0d75926aab93618902be)