bonding: update docs to correctly reflect arp_ip_target behavior

This documentation patch hopes to clarify that the '+' was only needed
for Fedora 7 and Red Hat Enterprise Linux 5.0 and 5.1.  After that the
IP addreses could be added as a comma separated list just like the
module option.

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>