[css-line-grid] match-parent creates new line grid for orthoganal flows

Author: astearns

css-line-grid/Overview.bs

@@ -225,7 +225,12 @@ Defining a <dfn>Line Grid</dfn>: the 'line-grid' property</h2>
 
 	<dl dfn-type="value" dfn-for="line-grid">
 		<dt><dfn>match-parent</dfn>
-		<dd>Box assumes the <a>line grid</a> of its parent.
+		<dd>Box assumes the <a>line grid</a> of its parent 
+			if its <a>writing mode</a> is the same as its parent. 
+			If the box has a <a>writing mode</a> 
+			that is different than its parent, 
+			then the box creates a new <a>line grid</a> 
+			as ''create'' below.
 
 		<dt><dfn>create</dfn>
 		<dd>Box creates a new <a>line grid</a>