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
Hi @tobibeer I wouldn't recommend people using `<pre>` tags in stylesheets. It's really confusing that TW doesn't implement the usual semantics of HTML `<pre>` tags.