[css-containment] Reuse existing terminology (#596)

Closes #396

Author: frivoal

css-containment/Overview.bs

@@ -221,10 +221,7 @@ Layout Containment</h3>
 				it must instead compute to ''continue/overflow''.
 
 	3. If the contents of the element overflow the element,
-		they must not project any "geometry" beyond the element's normal bounds.
-		In particular, any overflow must not be capable of causing the element's ancestors to overflow;
-		as far as the ancestors are concerned,
-		the element is perfectly confined to its normal width×height bounds.
+		they must be treated as <a>ink overflow</a>.
 
 	Possible optimizations that can be enabled by <a>layout containment</a> include (but are not limited to):