[css-scrollbars-1] Editorial clarification

Explain why the UA is allowed to change how it renders scrollbars in
order to color them.

See w3c#6443

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -123,10 +123,16 @@ which is generally fixed regardless of the scrolling position.
 Thumb refers to the moving part of the scrollbar,
 which usually floats on top of the track.
 
-If this property computes to value other than ''scrollbar-color/auto'',
+If this property computes to a value other than ''scrollbar-color/auto'',
 implementations may render a simpler scrollbar than the default platform UI rendering,
 and color it accordingly.
 
+Note: Sometimes the UA is unable to customize the colors of native scrollbars,
+perhaps due to how they're structured,
+or to a lack of control given by the native toolkit.
+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>)