outdated and inacurrate comment

[godsarmy]

i start a container with only pid namespace and i find when runc is killed, subprocesses are not always waited unless --no-subreaper is applied,

Therefore the comment of https://github.com/opencontainers/runc/blob/main/libcontainer/init_linux.go#L525 is outdated and inaccurate. If you agree, i can submit a PR to correct the comment.