[css-grid-3] Add behavior for stacking-axis self-alignment, per WG resolution. w3c#10275

Author: tabatkins

css-grid-3/Overview.bs

@@ -1119,6 +1119,22 @@ Alignment and Spacing</h2>
 	the [=grid container=]'s [=content box=] in the [=stacking axis=],
 	then the [=stacking range=] will be larger than the [=grid container=]'s [=content box=].
 
+	In the [=stacking axis=],
+	the [=self-alignment properties=] only apply to items that are adjacent to a gap in the layout,
+	i.e. placed in their [=grid track=](s) either immediately before a spanning item
+	or as the last item.
+	The [=alignment subject=] is the item's [=margin box=],
+	and the [=alignment container=] is that box plus the adjacent gap.
+
+	<figure>
+		<img src="images/align-self.png">
+		<figcaption>
+			The three highlighted items are the only ones with "gaps" following them,
+			so they're the only ones that will respond to 'align-items'.
+			Their [=alignment containers=] are indicated with the hatched areas.
+		</figcaption>
+	</figure>
+
 
 
 	ISSUE(10275): Is this a reasonable definition for how the [=self-alignment properties=] should work in the [=stacking axis=]?