[css-line-grid] Make the dependency on line-height-step explicit.

Closes w3c#491

css-line-grid/Overview.bs

@@ -234,7 +234,8 @@ Defining a <dfn>Line Grid</dfn>: the 'line-grid' property</h2>
 
 		<dt><dfn>create</dfn>
 		<dd>Box creates a new <a>line grid</a>
-			using its own font and line layout settings.
+			using its own font and line layout settings,
+			including any adjustment to the line height caused by the 'line-height-step' property.
 			The <a>line grid</a> consists of a series
 			of horizontal lines corresponding
 			to all the baselines