[css-align-3] Add anchor-center alignment to the condition for the 'halfway-safe' alignment

Author: tabatkins

css-align-3/Overview.bs

@@ -900,7 +900,9 @@ Self-Alignment for Absolutely Positioned Boxes</h5>
 	For [=absolutely positioned=] [=alignment subjects=] whose
 
 	* relevant [=self-alignment property=] is not ''justify-self/normal'' and
-	* whose 'inset-area' property successfully defines an [=inset-area grid=]
+	* whose 'inset-area' property successfully defines an [=inset-area grid=],
+		or whose [=self-alignment property=] is ''anchor-center''
+		(aka, the element is "using" Anchor Positioning in some way)
 
 	the default [=overflow alignment=] behavior is as follows: