[css-anchor-position-1] Fix the default alignment for a 2-region inset-area. w3c#10313

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -660,9 +660,11 @@ The <<inset-area>> also implies a default [=self-alignment=],
 which will be used if the [=self-alignment property=] on the element
 is ''align-self/normal'':
 
-* If the inset area includes the center region in an axis
+* If the only the center region in an axis is selected,
+	or all three regions are selected,
 	the default alignment in that axis is ''align-self/anchor-center''.
-* Otherwise, it's the opposite of the region it specifies:
+* Otherwise, the default alignment in that axis
+	is toward the non-specified side region:
 	if it's specifying the "start" region of its axis,
 	the default alignment in that axis is ''align-self/end''; etc.
 
@@ -671,7 +673,10 @@ is ''align-self/normal'':
 	then the value ''span-x-start top'' resolves to
 	the "start" region of the vertical axis,
 	and the "start" and "center" regions of the horizontal axis,
-	so the default alignments will be ''align-self: end;'' and ''justify-self: anchor-center;''
+	so the default alignments will be ''align-self: end''
+	(making the element's bottom flush with the bottom of the "top" region)
+	and ''justify-self: end''
+	(making the element's end side flush with the end side of the "start+center" region).
 
 	<figure>
 		<img src="images/inset-area-example.png" width=400>