[css-multicol-1] Don't export overly-generic terms

Author: fantasai

css-multicol-1/Overview.bs

@@ -1084,8 +1084,8 @@ Spanning Columns</h2>
 		</dd>
 	</dl>
 
-	An element that spans more than one column is called a <dfn export>spanning element</dfn>
-	and the box it creates is called a <dfn export>spanner</dfn>.
+	An element that spans more than one column is called a <dfn export local-lt="spanning element">multi-column spanning element</dfn>
+	and the box it creates is called a <dfn export local-lt="spanner">multi-column spanner</dfn>.
 
 	The [=containing block=] of the [=spanner=] is the [=multicol container=] itself.
 	Consequently, in cases where the spanner itself does not establish