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
I think it is very confusing indeed to find that this is possible and that people use it... I can imagine that it will lead to more of the questions as in the above discussion, unless some official spot documents that it works and how.

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.

Otherwise, I think it shouldn't be allowed.