Document main_menu state machine, name window constants

The new names for the constants (and newly named functions) give a much
better understanding of what the window system is trying to do.

src/field_message_box.c

@@ -127,7 +127,7 @@ static void textbox_auto_and_task_add(void)
 void HideFieldMessageBox(void)
 {
     task_del_textbox();
-    sub_8197434(0, 1);
+    ClearWindowAndWideBorder(0, 1);
     sFieldMessageBoxMode = 0;
 }