[css-grid] Reference Flexbox's a11y section on 'order'.

Author: fantasai

css-grid/Overview.bs

@@ -750,6 +750,14 @@ Reordered Grid Items: the 'order' property</h3>
 	The 'order' property also applies to <a>grid items</a>.
 	It affects their <a href="#grid-auto-flow-property">auto-placement</a> and <a href="#z-order">painting order</a>.
 
+	Advisement: As with reordering flex items,
+	the 'order' property must only be used
+	when the visual order needs to be <em>out-of-sync</em>
+	with the speech and navigation order;
+	otherwise the underlying document source should be reordered instead.
+	See <a href="https://drafts.csswg.org/css-flexbox-1/#order-accessibility">Reordering and Accessibility</a>
+	in [[CSS-FLEXBOX-1]].
+
 <h3 id='z-order'>
 Z-axis Ordering: the 'z-index' property</h3>