[css-sizing-4] Clarify that we're stretching the principal box w3c#11489

Author: fantasai

css-sizing-4/Overview.bs

@@ -834,8 +834,10 @@ Stretch-fit Sizing: filling the containing block</h3>
 
 	Stretch-fit sizing tries to set the box’s used size
 	to the length necessary
-	to make its outer size as close to filling the [=containing block=] as possible
-	while still respecting the constraints imposed by min-height/min-width/max-height/max-width.
+	to make its [=principal box=]’s [=outer size=]
+	as close to filling the [=containing block=] as possible
+	while still respecting the constraints imposed by
+	'min-height'/'min-width'/'max-height'/'max-width'.
 
 	: If used in an axis where the relevant [=self-alignment property=] applies to the element
 	::