[css-multicol] spanners become a formatting context, not always a BFC

Closes w3c#1071

css-multicol/Overview.bs

@@ -944,8 +944,8 @@ Spanning columns</h2>
 			ancestor in the same block formatting context. The element spans
 			across all columns. Content in the normal flow that appears before the
 			element is automatically balanced across all columns before the
-			element appears. The element establishes a new block formatting
-			context.
+			element appears.
+			The element <a>becomes a formatting context</a>.
 
 			Note: The element establishing a formatting content does not
 			depend of whether the element is a descendent of a multicol or not.
@@ -1724,6 +1724,7 @@ This appendix describes changes from the
 <ul>
 	<li>TBD
 	<li>Clarify that column-span causes the element to become a formatting context even if it is not in a multicol
+	<li>Column spanners do not always establish a <em>block</em> formatting context, but instead <a>become a formatting context</a>.
 </ul>
 
 <h2 class=no-num id=acknowledgments>