[css-overflow-3] Define nearest scrollport for convenience.

Author: fantasai

css-overflow-3/Overview.bs

@@ -323,6 +323,8 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 	(through which the <a>scrollable overflow area</a> can be viewed)
 	coincides with its padding box,
 	and is called the <dfn export>scrollport</dfn>.
+	For convenience, a box’s <dfn>nearest scrollport</dfn>
+	is the [=scrollport=] of its nearest [=scroll container=] ancestor.
 
 	<pre class=propdef>
 		Name: overflow-x, overflow-y