[css-overflow-3] Don't accept <'block-ellipsis'> value in -webkit-line-clamp. (#3695)

Author: heycam

css-overflow-3/Overview.bs

@@ -998,7 +998,11 @@ Legacy compatibility</h4>
 	For compatibility with legacy content,
 	UAs that support 'line-clamp' must also support the <dfn dfn-type=property>-webkit-line-clamp</dfn> property.
 	Like 'line-clamp', it is a shorthand of 'max-lines', 'continue', and 'block-ellipsis',
-	except that it sets 'continue' to ''-webkit-discard'' instead of ''discard''.
+	except that:
+
+	* its syntax is ''line-clamp/none'' | <<integer>>
+	* it sets 'continue' to ''-webkit-discard'' instead of ''discard''
+	* it unconditionally sets 'block-ellipsis' to ''block-ellipsis/auto''
 
 	Additionally, for children (including anonymous children)
 	of boxes whose 'display' property computes to ''-webkit-box'' or ''-webkit-inline-box'',