extend description of net.inet.ip.fw.one_pass

Description of net.inet.ip.fw.one_pass tunable refers only to dummynet(4),
while in reality is applicable on any divert-like packet action like
in-kernel nat, netgraph, reass, or similar.

Reviewed by: ae
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D51436