[css-conditional-5][editorial] Export 'size feature'

Author: tabatkins

css-conditional-5/Overview.bs

@@ -878,7 +878,7 @@ Size Container Features</h3>
 	allows querying
 	the size of the [=query container=]’s [=principal box=].
 	It is a boolean combination of
-	individual <dfn>size features</dfn> (<<size-feature>>)
+	individual <dfn export>size features</dfn> (<<size-feature>>)
 	that each query a single, specific dimensional feature of the [=query container=].
 	The syntax of a <dfn><<size-feature>></dfn> is the same as for a [=media feature=]:
 	a feature name, a comparator, and a value.