Document the new OS class support in the examples

[annegentle]

I don't quite understand yet how it's supposed to work - trying it in byod.html right now, and I'm still seeing Windows output on a Mac, from this source:

<div>
	<p>To work on this lab with your computer, you need to meet the requirements listed here. If you need to write instructions for one operating system you can use class settings for `mac`, `windows`, or `linux` surrounding the text you want displayed for a particular operating system.</p>
	<div class="mac linux">
	  <h3 class="linux mac">Mac OS X and Linux pre-requisites</h3>
	  <p> This is a paragraph.</p>
	  <ul>
		  <li>Bulleted List 1</li>
		  <li>Bulleted List 2</li>
		  <li>Bulleted List 3</li>
	  </ul>
  </div>
	<div class="windows">
	  <h3>Windows pre-requisites</h3>
	  <p>To use a hyperlink click <a href=https://www.cisco.com>here</a></p>
  </div>
</div>

[annegentle]

This feature doesn't work quite as expected yet.