[css-position-3] Clarify that the IMCB size is floored to 0. w3c#11478

Author: tabatkins

css-position-3/Overview.bs

@@ -755,7 +755,7 @@ Resolving ''inset/auto'' Insets</h4>
 <h4 id="resolving-inset-overconstraint">
 Resolving Overconstrained Insets</h4>
 
-	If the effective [=containing block=] size in any axis is less than zero,
+	If the [=inset-modified containing block=] size in any axis is less than zero,
 	then the [=weaker inset=] in the affected axis
 	is reduced (possibly becoming negative)
 	to bring that size up to zero.