[css-lists-3] Add a note to 'implicit list-item counter' pointing back to section 3.4, which details how the list-item counter is used by default. Fixes #4168.

Author: tabatkins

css-lists-3/Overview.bs

@@ -1087,6 +1087,14 @@ The Implicit ''list-item'' Counter</h3>
 	For example, HTML lists can have a <{ol/reversed}> attribute,
 	which causes the ''list-item'' counter to count <em>down</em> instead on the list items belonging to that element.
 
+	Note: Per [[#text-markers]],
+	the ''list-item'' counter
+	is used to generate the [=marker string=] for [=list items=];
+	in other words,
+	it's the counter used when you set 'list-style-type'
+	to something that can show a counter's value,
+	like ''decimal''.
+
 	<div class="example">
 		In the following example,
 		the list is modified to count by twos: