Add an example explaining how to use counters in alt text (w3c#12474)

Explain how to set up a fallback when using counter in alt text
in unsupported browsers.

Requested in I2S process:
https://groups.google.com/a/chromium.org/g/blink-dev/c/Ec36bxU5lok/m/5ljOQJS6BQAJ

Author: danielsakhapov

css-content-3/Overview.bs

@@ -210,6 +210,19 @@ Inserting and Replacing Content: the 'content' property</h2>
 			the element has no "alt text".
 	</dl>
 
+	<div class="example">
+		To correctly use <<counter>> in "alt text" in unsupported browsers
+		one should specify 'content' property twice.
+		First, a fallback without counter in "alt text".
+		Second, use counter in "alt text".
+		<pre>
+			::before {
+				content: "Chapter" counter(chapter);
+				content: "Chapter" counter(chapter) / "Chapter" counter(chapter);
+			}
+		</pre>
+	</div>
+
 	Issue: Should the contents keyword be replaced with ''content()''?
 
 <h3 id="accessibility">
@@ -1397,4 +1410,4 @@ No new privacy considerations have been reported on this specification.
 
 <h2 class=no-num id=security>Security Considerations</h2>
 
-No new security considerations have been reported on this specification.
+No new security considerations have been reported on this specification.