document the ramifications of submenus better

[jidanni]

On
(info "(grub) default")
say if
   "Menu Entry 3" would then be identified as `Submenu 2>Submenu 3>Menu Entry 3'
can also be identified by
   "Menu Entry 3" would then be identified as `Submenu 2>Submenu 3>3'.

And also say why if you are counting the menu entries, you still require
the user to add all the submenus.

Also (info "(grub) Simple configuration") does not mention submenus nor
even link to (info "(grub) default").

(GRUB) 2.00-15