useradd help says it "Adds a native user" and a file based user

*Original comment by @LeeDr:*

Running 5.0.0 alpha2;

```
EMAIL REDACTED /usr/share/elasticsearch/bin/x-pack/users useradd -h
Adds a native user

Adds a file based user to elasticsearch (via internal realm). The user will...
```

From here (I think);
LINK REDACTED


