[DOC] Move the internal document for `Init_class_hierarchy`

It has hidden the document for `Object` class.

class.c

@@ -803,6 +803,27 @@ refinement_import_methods(int argc, VALUE *argv, VALUE refinement)
 }
 # endif
 
+/*!
+ *--
+ * \private
+ * Initializes the world of objects and classes.
+ *
+ * At first, the function bootstraps the class hierarchy.
+ * It initializes the most fundamental classes and their metaclasses.
+ * - \c BasicObject
+ * - \c Object
+ * - \c Module
+ * - \c Class
+ * After the bootstrap step, the class hierarchy becomes as the following
+ * diagram.
+ *
+ * \image html boottime-classes.png
+ *
+ * Then, the function defines classes, modules and methods as usual.
+ * \ingroup class
+ *++
+ */
+
 void
 Init_class_hierarchy(void)
 {

object.c

@@ -4173,27 +4173,6 @@ f_sprintf(int c, const VALUE *v, VALUE _)
  *
  */
 
-/*!
- *--
- * \private
- * Initializes the world of objects and classes.
- *
- * At first, the function bootstraps the class hierarchy.
- * It initializes the most fundamental classes and their metaclasses.
- * - \c BasicObject
- * - \c Object
- * - \c Module
- * - \c Class
- * After the bootstrap step, the class hierarchy becomes as the following
- * diagram.
- *
- * \image html boottime-classes.png
- *
- * Then, the function defines classes, modules and methods as usual.
- * \ingroup class
- *++
- */
-
 void
 InitVM_Object(void)
 {