[css-transforms-1] Add a paragraph to Module Interactions to reference computation of scrollable overflow (#914).

Author: smfr

css-transforms-1/Overview.bs

@@ -102,6 +102,8 @@ Transforms affect the rendering of backgrounds on elements with a value of ''fix
 
 Transforms affect the client rectangles returned by the Element Interface Extensions <a href="https://www.w3.org/TR/cssom-view/#dom-element-getclientrects">getClientRects()</a> and <a href="https://www.w3.org/TR/cssom-view/#dom-element-getboundingclientrect">getBoundingClientRect()</a>, which are specified in [[CSSOM-VIEW]].
 
+Transforms affect the computation of the <a href-"https://www.w3.org/TR/css-overflow-3/#scrollable-overflow-region">scrollable overflow region</a> as described by [[CSS-OVERFLOW-3]].
+
 CSS Values {#css-values}
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