doc: Slight tweaks to wl_listener

Prefer \comment over // in code blocks for consistency's sake and keep
variable definitions separated by a line from the rest of the body.

src/wayland-server.h

@@ -143,17 +143,18 @@ wl_client_post_no_memory(struct wl_client *client);
  * listener should be done through provided accessor methods. A listener can
  * only listen to one signal at a time.
  *
- * \code
+ * ~~~
  * struct wl_listener your_listener;
+ *
  * your_listener.notify = your_callback_method;
  *
- * // Direct access
+ * \comment{Direct access}
  * wl_signal_add(&some_object->destroy_signal, &your_listener);
  *
- * // Accessor access
+ * \comment{Accessor access}
  * wl_event_loop *loop = ...;
  * wl_event_loop_add_destroy_listener(loop, &your_listener);
- * \endcode
+ * ~~~
  *
  * If the listener is part of a larger struct, #wl_container_of can be used
  * to retrieve a pointer to it:
@@ -162,6 +163,7 @@ wl_client_post_no_memory(struct wl_client *client);
  * void your_listener(struct wl_listener *listener, void *data)
  * {
  * 	struct your_data *data = NULL;
+ *
  * 	your_data = wl_container_of(listener, data, your_member_name);
  * }
  * \endcode