[css-scrollbars] Mark issues as issues

Leaving issues as Notes makes then not stand out, and risk having people
forget to fix them.

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -139,11 +139,11 @@ Value Definitions</h3>
 	The provision above allows the UA to replace them with differently-constructed scrollbars,
 	which it does know how to color.
 
-	(Note: add diagram showing the different named pieces - something like
-	<a href="http://www.howtocreate.co.uk/tutorials/scrlbar.html">http://www.howtocreate.co.uk/tutorials/scrlbar.html</a>)
+	Issue: add diagram showing the different named pieces - something like
+	<a href="http://www.howtocreate.co.uk/tutorials/scrlbar.html">http://www.howtocreate.co.uk/tutorials/scrlbar.html</a>
 
-	(Note: add example of an overflow element with colorized scrollbars to match page styling,
-	PNG of the same in a browser that supports it currently)
+	Issue: add example of an overflow element with colorized scrollbars to match page styling,
+	PNG of the same in a browser that supports it currently.
 
 	Implementations may ignore any of the colors
 	if the corresponding part do not exist on the underlying platform.