Have CSS classes for section header and description

This simple patch makes it possible to style sections.

To understand why it's important, here's a "before" page that shows the current sad situation:

http://www.typo.co.il/~mooffie/tmp/before.html

This page looks like a long jumble of text. The user isn't likely to read the section descriptions because they aren't distinguishable from function items. It's a mess.

Now,

Here's an "after" page, whose CSS takes advantage of the ability to style section headers/descriptions:

http://www.typo.co.il/~mooffie/tmp/after.html

I think it's a major usability improvement.

(The CSS I used here is [this one](http://www.typo.co.il/~mooffie/tmp/mycss.txt). After I finish with this project I'll make my configuration files public.)

**Note**: The reason we output the `<div class="section-description">` only, and only if, there's actually a description is because a style like `.section-description {border: 1px solid green}` affects empty DIVs too and we want to void this.