http://bugzilla.redhat.com/601143, part 1 - document existing
behavior.  Ever since Mar 2010 (commit ced154cb), the use of
'attach-disk' or 'attach-device' to change cdrom/floppy media has been
documented but deprecated, but the replacement to use 'update-device'
was not documented.

* tools/virsh.c (cmdAttachInterface, cmdAttachDisk): Fix bad error
message.
* tools/virsh.pod (attach-device, attach-disk): Refer to
update-device for cdrom and floppy behavior.
(update-device): Add documentation.