Document the Markdown replacements of DocBook-style CSS classes

See http://lists.autistici.org/message/20181118.155100.286e6cc2.en.html.

We should remove this from our documentation guidelines.

From the W3C standard, I think that we should use <i> and <b> when writing full HMTL and not <em> and <strong>: http://html5doctor.com/i-b-em-strong-element/

But since ikiwiki uses <em> and <strong>, I think that we should also use them for consistency across the website.