and add comment

Refer to coreos#49 (comment),
do testing:
1. Remove bin line in group and passwd
2. Build FCOS, see logs:
```
systemd.post: Creating group 'bin' with GID 1.
systemd.post: Creating user 'bin' (bin) with UID 1 and GID 1.
systemd.post: /etc/gshadow: Group "bin" already exists.
```

According to @cgwalters 's pointer:

The above log will lead systemd-sysusers (during systemd.post)
exit early before saving the updated `/etc/{passwd,group}` refer
to [code](https://github.com/systemd/systemd/blob/main/src/sysusers/sysusers.c#L820),
and bin user/group will not be saved finally.

The root cause is that `gshadow` is not consistent with group,
`gshadow` is from setup, and we override group according to https://github.com/coreos/fedora-coreos-config/blob/testing-devel/manifests/group.

The `shadow` is also from setup, and is not consistent with
passwd, we should also sync it.

Fix coreos/fedora-coreos-tracker#1525

Refer to coreos#4503 (comment)