Clarify scrolling submenu behavior

These are observations and tests based on Atari's AES 4.1 and their own
newsletter article on menu_popup().

One can make an attached submenu scroll, it would look weird with a
scrollbar and Atari clearly knew this.

N.AES mimics this behavior exactly.

Contributed by Lonny Pursell.

gem/aes/structures/menu.ui

@@ -5,14 +5,15 @@
 !begin_verbatim
 typedef struct
 {
-    OBJECT  *mn_tree;    /* The object tree of the menu             */
-    int16_t mn_menu;     /* The parent object of the menu items     */
-    int16_t mn_item;     /* The starting menu item                  */
-    int16_t mn_scroll;   /* The scroll field status of the menu:    */
-                         /*  0  - The menu will not scroll          */
-                         /*  >0 - The menu will scroll              */
-    int16_t mn_keystate; /* The [CTRL], [ALT], [SHIFT] key state at */
-                         /* the time the mouse button was pressed   */
+    OBJECT  *mn_tree;    /* The object tree of the menu                 */
+    int16_t mn_menu;     /* The parent object of the menu items         */
+    int16_t mn_item;     /* The starting menu item                      */
+    int16_t mn_scroll;   /* The scroll field status of the menu:        */
+                         /*  0  - The menu will not scroll              */
+                         /*  >0 - The menu will scroll (no scrollbar)   */
+                         /*  -1 - The menu will scroll (with scrollbar) */
+    int16_t mn_keystate; /* The [CTRL], [ALT], [SHIFT] key state at     */
+                         /* the time the mouse button was pressed       */
 } MENU;
 !end_verbatim