[css-flexbox-1] Annotate flex column wrap sample fragmentatio algo as problematic

Author: fantasai

css-flexbox-1/Overview.bs

@@ -3347,7 +3347,8 @@ Sample Flex Fragmentation Algorithm</h3>
 			</ol>
 
 			<p class='note'>
-				If a flex item does not entirely fit on a single page,
+				A shortcoming of this sample algorithm is that
+				if a flex item does not entirely fit on a single page,
 				it will <em>not</em> be paginated in <a>multi-line</a> column flex containers.
 
 		<dt><a>single-line</a> row flex container