[css-multicol-1] Clarify that spanners interrupt column rules. w3c#2579

Author: fantasai

css-multicol-1/Overview.bs

@@ -1083,13 +1083,17 @@ Spanning Columns</h2>
 		</pre>
 
 		By setting 'column-span' to ''column-span/all'', 
-		all content that appears before the <code>h2</code> element 
-		is shown before the <code>h2</code> element.
+		all content that appears before the <code>h2</code> element
+		is shown above the <code>h2</code> element.
 
 		<figure>
 			<img src="images/h2-spanner.svg" alt="An element spans all three columns">
 			<figcaption>The h2 element is set to column-span: all</figcaption>
 		</figure>
+
+		Note that because the spanner splits the [=multi-column line=],
+		it also interrupts any [=column rules=]
+		(which are only drawn between [=columns=] in a [=multi-column line=]).
 	</div>
 
 	A spanning element may be lower than the first level of descendants