[css-grid] Clarify where the sole line is aligned when there are no tracks.

Author: tabatkins

css-grid/Overview.bs

@@ -3120,6 +3120,10 @@ Aligning the Grid: the 'justify-content' and 'align-content' properties</h3>
 		</figure>
 	</div>
 
+	If there are no <a>grid tracks</a>
+	(the <a>explicit grid</a> is empty, and no tracks were created in the <a>implicit grid</a>),
+	the sole <a>grid line</a> in each axis is aligned with the start edge of the <a>grid container</a>.
+
 	Note that certain values of 'justify-content' and 'align-content'
 	can cause the tracks to be spaced apart
 	(''justify-content/space-around'', ''justify-content/space-between'', ''justify-content/space-evenly'')