[css-grid] Add issue to replace example 21.

Author: tabatkins

css-grid/Overview.bs

@@ -1843,6 +1843,16 @@ The Implicit Grid</h2>
 	auto-placement will also cause the generation of <a>implicit grid tracks</a>.
 
 	<div class='example'>
+		Issue: This is a bad example and needs to be totally rewritten.
+		It doesn't actually use the grid-auto-* properties,
+		which is confusing;
+		the example rendering is confusing and doesn't quite match what it would actually do,
+		because the second column would just shrink to zero as well;
+		and now that auto tracks stretch by default,
+		it's <em>totally wrong</em>--
+		all the auto tracks will stretch to fill the space,
+		so columns 3 and 4 are definitely visible.
+
 		This example illustrates the sizing of <a>implicit grid tracks</a>.
 		Note that <a>grid item</a> B is positioned on <a>grid line</a> 5,
 		which automatically creates <strong>four</strong> <a>implicit grid columns</a>.