Update Using Stylesheets.tid

added documentation for html in StyleSheets

https://groups.google.com/d/msg/tiddlywiki/Qx0y7btud5M/Ch6hMnh0ud0J

simplified the specificity section, some rewording
> unless some official spot documents that it works and how.

The docs don't need to tell people how to use `<pre>` tags, they just need to explain how CSS parsing works, so that people understand that they can use HTML tags and wikitext formatting.

> If it's allowed, I don't see a reason not to recommend it, because it helps indeed to show those stylesheets in a meaningful way, which wouldn't be the case when no <pre> tags were used.

I agree that it would be good if the stylesheets displayed more readably, but we're already tackling that with #1189

> Otherwise, I think it shouldn't be allowed.

Well, it's difficult to disallow it; the ability to intersperse HTML tags follows as a consequence of building CSS out of wikitext.