[css-sizing-4] CSS2 ch 10 is non-exhaustive. Make it an example and add one more.

Author: fantasai

css-sizing-4/Overview.bs

@@ -84,7 +84,8 @@ Intrinsic Aspect Ratios: the 'aspect-ratio' property</h3>
 	The box’s <a>automatic sizes</a>
 	are then calculated the same as for
 	a <a>replaced element</a> with an <a>intrinsic aspect ratio</a>,
-	see <a href="https://www.w3.org/TR/CSS2/visudet.html">CSS2&sect;10</a>.
+	see e.g. <a href="https://www.w3.org/TR/CSS2/visudet.html">CSS2 &sect;&#x202F;10</a>
+	and <a href="https://www.w3.org/TR/css-flexbox-1/#algo-main-item">CSS Flexible Box Model Level 1 &sect;&#x202F;9.2</a>.
 	The axis in which the <a>preferred size</a> calculation
 	depends on this aspect ratio
 	is called the <dfn>ratio-dependent axis</dfn>,