[css-grid] Clarify context in early mention of grid areas

Author: fantasai

css-grid/Overview.bs

@@ -116,7 +116,7 @@ Background and Motivation</h3>
 	It provides a mechanism for authors to divide available space for layout into columns and rows
 	using a set of predictable sizing behaviors.
 	Authors can then precisely position and size the building block elements of their application
-	into <a>grid areas</a> defined by these columns and rows.
+	into <a>grid areas</a> defined by the intersection of these columns and rows.
 	Figure 1 illustrates a basic layout which can be achieved with grid layout.
 
 <h3 id='adapting-to-available-space'>