[css-will-change] Clarify that abspos-CB-generators also work, and use the right term for fixpos.

Author: tabatkins

css-will-change/Overview.bs

@@ -271,8 +271,11 @@ Hinting at Future Behavior: the 'will-change' property</h2>
 			If any non-initial value of a property would create a stacking context on the element,
 			specifying that property in 'will-change' must create a stacking context on the element.
 
-			If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements,
-			specifying that property in 'will-change' must cause the element to generate a containing block for fixed-position elements.
+			If any non-initial value of a property would cause the element to generate a containing block for absolutely positioned elements,
+			specifying that property in 'will-change' must cause the element to generate a containing block for absolutely positioned elements.
+
+			If any non-initial value of a property would cause the element to generate a containing block for fixed positioned elements,
+			specifying that property in 'will-change' must cause the element to generate a containing block for fixed positioned elements.
 
 			If a non-initial value of a property would cause rendering differences on the element
 			(such as using a different anti-aliasing strategy for text),