[css-images-4] Add two png images to example 33 to the stripes() function.

Author: francescorn

css-images-4/Overview.bs

@@ -2047,7 +2047,7 @@ Color Stop “Fixup”</h4>
 		the value of the sum of the <<flex>> values,
 		to obtain only 90px,
 		which is then distributed in the 1:2 ratio
-		dictated by the <<flex>> values.
+		dictated by the <<flex>> values. (Transparent is shown with a border in the example).
 
 		<pre class=lang-css>
 		<img src="images/stripes_example_33_2.png" style="height:200px; width:400px;" alt="">