[css-grid-3][editorial] Add a note clarifying the meaning of 'placement' wrt baseline positioning. w3c#13165

Author: tabatkins

css-grid-3/Overview.bs

@@ -1196,6 +1196,11 @@ Baseline Alignment</h3>
 	and the last baseline set from the lowest [=alignment baseline=]
 	among the [=grid items=] placed last in each track.
 
+	Note: Whether an item is “first” or “last” depends only on placement
+	(including ''grid-auto-flow/dense'' packing adjustments),
+	and is not affected by its actual position on the page,
+	e.g. negative margins or [=relative positioning=] are not considered.
+
 
 <h2 id="pagination">
 Fragmentation</h2>