timeout(1): Kill self with the same signal that terminated the child

A shell may not set '$?' to '128 + signal_number' when the process was
terminated by a signal. For example, KornShell 93 sets '$?' to
'256 + signal_number' in such cases. In order to avoid any possible
ambiguity, the POSIX.1-2024 standard requires that timeout mimic the
wait status of the child process by terminating itself with the same
signal, while disabling core generation.

Update the man page accordingly.

Obtained-from: DragonFly BSD
Reference: https://pubs.opengroup.org/onlinepubs/9799919799/utilities/timeout.html