[css-grid] Define the explicit grid even when there are no tracks.

Author: tabatkins

css-grid/Overview.bs

@@ -914,12 +914,15 @@ The Explicit Grid</h2>
 	and the number of rows/columns sized by 'grid-template-rows'/'grid-template-columns'.
 	Any rows/columns defined by 'grid-template-areas' but not sized by 'grid-template-rows'/'grid-template-columns'
 	take their size from the 'grid-auto-rows'/'grid-auto-columns' properties.
+	If these properties don't define <em>any</em> <a>explicit</a> tracks,
+	the <a>explicit grid</a> still contains one <a>grid line</a> in each axis.
 
 	Numeric indexes in the <a>grid-placement properties</a>
-	count from the edges of the <a>explicit grid</a>,
-	starting from 1.
-	Positive indexes count from the before/start side,
-	while negative indexes count from the after/end side.
+	count from the edges of the <a>explicit grid</a>.
+	Positive indexes count from the <a>start</a> side
+	(starting from 1 for the <a>start</a>-most <a>explicit</a> line),
+	while negative indexes count from the <a>end</a> side
+	(starting from -1 for the <a>end</a>-most <a>explicit</a> line).
 
 <h3 id='track-sizing'>
 Track Sizing: the 'grid-template-rows' and 'grid-template-columns' properties</h3>