[css-text-3] Add missing sentence defining what grid-area is a shorthand for. w3c#3492

Author: fantasai

css-grid-1/Overview.bs

@@ -2995,6 +2995,9 @@ Placement Shorthands: the 'grid-column', 'grid-row', and 'grid-area' properties<
 	Animation type: discrete
 	</pre>
 
+	The 'grid-area' property is a <a>shorthand</a> for
+	'grid-row-start', 'grid-column-start', 'grid-row-end' and 'grid-column-end'.
+
 	If four <<grid-line>> values are specified,
 	'grid-row-start' is set to the first value,
 	'grid-column-start' is set to the second value,