[css-will-change] Specify that custom properties don't trigger the side-effects.

Author: tabatkins

css-will-change/Overview.bs

@@ -256,6 +256,9 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 			It is still <em>safe</em> to specify them, though;
 			it'll simply have no effect.
 
+			Specifying a <a>custom property</a> must have no effect,
+			including the "side-effect" management detailed below.
+
 			<p class='example'>
 				For example, browsers often handle elements with 'transform' set to a non-initial value very differently from normal elements,
 				perhaps rendering them to their own “GPU layer”