[css-multicol-1] Export dfn of “multi-column layout”, parallel to “flex layout” and “grid layout”.

Author: fantasai

css-multicol-1/Overview.bs

@@ -46,7 +46,7 @@ Introduction</h2>
 
 	(This section is not normative.)
 
-	This module describes multi-column layout in CSS. By using
+	This module describes <dfn export>multi-column layout</dfn> in CSS. By using
 	functionality described in this document, style sheets can declare
 	that the content of an element is to be laid out in multiple
 	columns.