The last part of the sentence was accidentally put in enumeration, It
made an impression that it's one of the rules, not the result of
applying the rules.

Replacing `...` with `*pids` seems to clarify the expected variadic arguments.

Note that the expected arguments are two or more with a signal and pids.
That is, the method must have at least one pid, which cannot be omitted:

```console
% ruby -e 'Process.kill(0)'
-e:1:in `kill': wrong number of arguments (given 1, expected 2+) (ArgumentError)
        from -e:1:in `<main>'
```

The forked child process is a grandchild process from the viewpoint of
the process which invoked the caller process.  That means the child is
detached at that point, and it does not need to fork twice.

First, rb_mjit_fork should call rb_thread_atfork to stop threads after
fork in the child process. Unfortunately, we cannot use rb_fork_ruby to
prevent this kind of mistakes because MJIT needs special handling of
waiting_pid and mjit_pause/resume.

Second, mjit_waitpid_finished should be checked regardless of
trap_interrupt. It doesn't seem like the flag is not set when SIGCHLD is
handled for an MJIT child process.

This takes care of signal_self_pipe and other things.