docs: Move the GMenu SECTION

Move it to the struct docs.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3037

gio/gmenu.c

@@ -28,27 +28,17 @@
 
 #include "gicon.h"
 
-/**
- * SECTION:gmenu
- * @title: GMenu
- * @short_description: A simple implementation of GMenuModel
- * @include: gio/gio.h
- *
- * #GMenu is a simple implementation of #GMenuModel.
- * You populate a #GMenu by adding #GMenuItem instances to it.
- *
- * There are some convenience functions to allow you to directly
- * add items (avoiding #GMenuItem) for the common cases. To add
- * a regular item, use g_menu_insert(). To add a section, use
- * g_menu_insert_section(). To add a submenu, use
- * g_menu_insert_submenu().
- */
-
 /**
  * GMenu:
  *
- * #GMenu is an opaque structure type.  You must access it using the
+ * `GMenu` is a simple implementation of [class@Gio.MenuModel].
- * functions below.
+ * You populate a `GMenu` by adding [class@Gio.MenuItem] instances to it.
+ *
+ * There are some convenience functions to allow you to directly
+ * add items (avoiding [class@Gio.MenuItem]) for the common cases. To add
+ * a regular item, use [method@Gio.Menu.insert]. To add a section, use
+ * [method@Gio.Menu.insert_section]. To add a submenu, use
+ * [method@Gio.Menu.insert_submenu].
  *
  * Since: 2.32
  */