[css-grid][css-flexbox] Clarify that z-index on flex items and grid items behaves exactly as if position were relative because this apparently needs clarification. Fixes w3c#1312

Author: fantasai

css-flexbox/Overview.bs

@@ -741,7 +741,8 @@ Flex Item Z-Ordering</h3>
 	except that 'order'-modified document order is used in place of raw document order,
 	and 'z-index'
 	values other than ''z-index/auto'' create a stacking context
-	even if 'position' is ''static''.
+	even if 'position' is ''static''
+	(behaving exactly as if 'position' were ''relative'').
 
 	Note: Descendants that are positioned outside a flex item still participate
 	in any stacking context established by the flex item.

css-grid/Overview.bs

@@ -1175,7 +1175,8 @@ Z-axis Ordering: the 'z-index' property</h3>
 	or even when positioned in non-intersecting areas because of negative margins or positioning.
 	The painting order of <a>grid items</a> is exactly the same as inline blocks [[CSS21]],
 	except that <a>order-modified document order</a> is used in place of raw document order,
-	and 'z-index' values other than <a value for=z-index>auto</a> create a stacking context even if 'position' is ''static''.
+	and 'z-index' values other than <a value for=z-index>auto</a> create a stacking context even if 'position' is ''static''
+	(behaving exactly as if 'position' were ''relative'').
 	Thus the 'z-index' property can easily be used to control the z-axis order of grid items.
 
 	<p class='note'>