Clarify net.inet.ip.allow_net240 and allow_net0

The stack has never limited use of addresses in these ranges as an
endpoint. The relatively recent sysctls control only forwarding of,
and ICMP response to, these addresses.

Reviewed by: bz
Fixes: efe58855f3ea ("IPv4: experimental changes to allow net 0/8, 240/4, part of 127/8")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48262