Include scroll bar arrows in examples.

As per #3752

Author: egirard

css-scroll-snap-1/Overview.bs

@@ -917,7 +917,7 @@ Types of Scrolling Methods {#scroll-types}
         <div class="example">
             Common examples of scrolls with only an <a>intended direction</a> include:
 
-            * pressing an arrow key on the keyboard
+            * pressing an arrow key on the keyboard (or equivalent operations on the scrollbar)
             * a swiping gesture interpreted as a fixed (rather than inertial) scroll
         </div>