[css-overflow-3] Clarify to avoid misunderstanding

Addresses the comment in w3c#1971 (comment)

Author: frivoal

css-overflow-3/Overview.bs

@@ -384,7 +384,7 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 		</dl>
 
 	If the computed value of 'overflow' on a <a>block box</a>
-	is neither ''overflow/visible'' nor ''overflow/clip'',
+	is neither ''overflow/visible'' nor ''overflow/clip'' nor a combination thereof,
 	it creates a new <a>block formatting context</a> for its content.