[css-grid] Add issues for additional grid-auto-flow values, per WG resolution.

Author: tabatkins

css-grid/Overview.html

@@ -2404,6 +2404,10 @@ <h3 id=grid-auto-flow-property><span class=secno>6.3. </span> Automatic
     turn, adding new columns as necessary.
   </dl>
 
+  <p class=issue> Add a value for stacking items on top of each other?
+
+  <p class=issue> Add a value for flowing items together into a single slot?
+
   <p> Auto-placement takes <a href="#grid-item"><i>grid items</i></a> in
    ‘<a href="#order"><code class=property>order</code></a>’-modified
    document order.

css-grid/Overview.src.html

@@ -1852,6 +1852,12 @@ <h3 id="grid-auto-flow-property">
 			adding new columns as necessary.
 	</dl>
 
+	<p class='issue'>
+		Add a value for stacking items on top of each other?
+
+	<p class='issue'>
+		Add a value for flowing items together into a single slot?
+
 	<p>
 		Auto-placement takes <i>grid items</i> in 'order'-modified document order.