[css-overflow-3] Update intro

Author: fantasai

css-overflow-3/Overview.bs

@@ -91,11 +91,12 @@ Introduction</h2>
 	which makes sense when the author's intent
 	is that the content not be shown.
 
-	This specification introduces the long-standing de-facto 'overflow-x' and 'overflow-y' properties,
+	This specification defines the long-standing de-facto 'overflow-x' and 'overflow-y' properties,
 	adds a ''overflow/clip'' value,
+	introduces an 'overflow-clip-margin' property,
 	and defines overflow handling more fully.
-
-	[Something something 'max-lines'.]
+	It also introduces control over smooth scrolling via 'scroll-behavior'
+	and for reserving space for the scrollbar via 'scrollbar-gutter'.
 
 	Note: This specification also reproduces the definition of the 'text-overflow' property
 	previously defined in [[CSS-UI-3]],