docs: update info about code block with class

docs/definitions.md

@@ -220,6 +220,18 @@ results in
 <pre><code>
 some code
 </code></pre>
+```
+
+    ```cpp
+    int a = 42;
+    ```
+
+results in
+
+```html
+<pre class="cpp"><code>
+int a = 42;
+</code></pre>
 ```
 
 ## Inline code