[css-position-3] Add dfn for inset-modified containing block to help with abspos definitions.

Author: fantasai

css-position-3/Overview.bs

@@ -628,7 +628,7 @@ Absolute positioning</h3>
 	is reduced to bring that size up to zero.
 
 	Sizing and positioning of the [=absolutely positioned box=]
-	into this effective [=containing block=]
+	into this <dfn>inset-modified containing block</dfn>
 	is as described in [[#size-and-position-details]].
 
 <h4 id="breaking">