[css-inline-3] Define ideographic baseline properly.

Author: fantasai

css-inline-3/Overview.bs

@@ -103,7 +103,8 @@ Dominant Baselines: the 'dominant-baseline' property</h3>
 
 		<dt><dfn>ideographic</dfn>
 		<dd>
-			Match the box's ideographic character face under-side baseline to that of its parent.
+			Match the box's ideographic under-side baseline to that of its parent.
+			This corresponds to the <code>ideo</code> baseline in OpenType.
 
 		<dt><dfn>middle</dfn>
 		<dd>