[css-lists-3] Add example of manipulating list-item counter.

Author: fantasai

css-lists-3/Overview.bs

@@ -786,7 +786,7 @@ Manipulating Counters: the 'counter-increment', 'counter-set' and 'counter-reset
 	</div>
 
 <h3 id="list-item-counter">
-The special ''list-item'' counter</h3>
+The Implicit ''list-item'' Counter</h3>
 
 	<a>List items</a> automatically increment
 	a special <dfn value for="counter-increment, counter-set, counter-reset, counter(), counters()">list-item</dfn> <a>counter</a>.
@@ -796,6 +796,20 @@ The special ''list-item'' counter</h3>
 	at the same time that <a>counters</a> are normally incremented.
 	(This has no effect on the computed values of the 'counter-*' properties.)
 
+	<div class="example">
+		In the following example,
+		the list is modified to count by twos:
+		<pre>
+		  ol.evens { counter-increment: list-item 2; }
+		</pre>
+		A three-item list would be rendered as
+		<pre>
+		  2. First Item
+		  4. Second Item
+		  6. Third Item
+		</pre>
+	</div>
+
 	Issue: More definition.
 	See old <a href="https://github.com/w3c/csswg-drafts/issues/2464#issuecomment-375492907">WG discussions</a>
 	and newer <a href="https://github.com/w3c/csswg-drafts/issues/3686">issue discussion</a>.