The net.inet.tcp.inflight* sysctls are in their own, separate sysctl
node since revision 1.196 of `src/sys/netinet/tcp_subr.c'. Update
their description in the Handbook, but keep a reference to the old
pre-5.3-RELEASE names until we stop supporting 4.X releases (a couple
of years from now).

PR: docs/76462
Submitted by: Joel Dahl <joel@automatvapen.se>