Clarify menu_attach()

Fixes #173.

gem/aes/menu_/menu_attach.ui

@@ -46,6 +46,15 @@ MENU structure of the submenu
 created for a menu entry, though up to (!I)four(!i) levels are supported. The
 number of submenus per menu is restricted to 64.
 
+If a menu item is expected to have a submenu attachment, the G_STRING must be
+padded with blanks to the width of the menu.
+
+Attaching a submenu to a menu item is done by calling menu_attach(). A submenu
+is associated to a menu item by placing a right arrow two characters in front
+of the right edge. Any character at that location will be overwritten.
+
+When detaching a submenu the right arrow will be overwritten with a blank.
+
 The (!I)ob_x(!i) and (!I)ob_y(!i) fields of the root object should always be set
 to 0 before making a menu_attach call.