[DOC] Mention the case to autoload already defined constant

load.c

@@ -1313,6 +1313,10 @@ ruby_init_ext(const char *name, void (*init)(void))
  *     end
  *     A.autoload(:B, "b")
  *     A::B.doit            # autoloads "b"
+ *
+ * If _const_ in _mod_ is defined as autoload, the file name to be
+ * loaded is replaced with _filename_.  If _const_ is defined but not
+ * as autoload, does nothing.
  */
 
 static VALUE
@@ -1373,6 +1377,10 @@ rb_mod_autoload_p(int argc, VALUE *argv, VALUE mod)
  *  a symbol) is accessed.
  *
  *     autoload(:MyModule, "/usr/local/lib/modules/my_module.rb")
+ *
+ * If _const_ is defined as autoload, the file name to be loaded is
+ * replaced with _filename_.  If _const_ is defined but not as
+ * autoload, does nothing.
  */
 
 static VALUE