Bug#586462: Description fails to mention USB though image is provided

[jidanni at jidanni.org]

Package: grub-rescue-pc

Many users remove this package because they think "floppy? CD-ROM? My
computer doesn't even have those drives." When in fact all along a USB
image is included but you forgot to mention it here:

Description: GRUB bootable rescue images, version 2 (PC/BIOS version)
 This package contains two GRUB rescue images that have been built for use with
                       ^^^three
 traditional PC/BIOS architecture:

 * grub-rescue-floppy.img: floppy image.
 * grub-rescue-cdrom.iso: El Torito CDROM image.
Also mention the third USB image.

Perhaps mention that they are all the same image in fact.