[css-sizing-4] Clarify referring to in-flow phantom content.

Author: fantasai

css-sizing-4/Overview.bs

@@ -413,7 +413,7 @@ Overriding Contained Intrinsic Sizes: the 'contain-intrinsic-size' property</h3>
 
 	This property allows elements with [=size containment=] to specify
 	an <dfn export>explicit intrinsic inner size</dfn>,
-	causing the box to size as if its content
+	causing the box to size as if its in-flow content
 	totals to a width and height
 	matching the specified [=explicit intrinsic inner size=]
 	(rather than sizing as if it were empty).