[css-transforms-2] Mention scroll offsets.

Mention scroll offsets, since they're an important source of offsets,
particularly for the 'perspective' property.

Fixes w3c#1298

Author: dbaron

css-transforms-2/Overview.bs

@@ -466,7 +466,7 @@ The final value of the transform used to render an element in a <a>3D rendering
 4. While <var>current element</var> is not the element that establishes the transformed element's <a>3D rendering context</a>:
 
 		1. If <var>current element</var> has a value for 'transform' which is not ''transform/none'', pre-multiply <var>current element</var>'s <a>transformation matrix</a> with the <var>transform</var>.
-		2. Compute a translation matrix which represents the offset of <var>current element</var> from its <var>parent element</var>, and pre-multiply that matrix into the <var>transform</var>.
+		2. Compute a translation matrix which represents the offset (including the scroll offset) of <var>current element</var> from its <var>parent element</var>, and pre-multiply that matrix into the <var>transform</var>.
 		3. If <var>parent element</var> has a value for 'perspective' which is not ''perspective/none'', pre-multiply the <var>parent element</var>'s <a>perspective matrix</a> into the <var>transform</var>.
 		4. Let <var>current element</var> be the <var>parent element</var>.
 		5. Let <var>parent element</var> be the <var>current element</var>'s parent.