Confused about cluster setting kv.closed_timestamp.target_duration in v21.2.10

Exalate commented:

I am running a self-hosted v21.2.10. In the db console I go to https://localhost:8080/#/reports/settings and see a row for this setting:

```
kv.closed_timestamp.target_duration | 3s | if nonzero, attempt to provide closed timestamp notifications for timestamps trailing cluster time by approximately this duration
```

But on the documentation page at https://www.cockroachlabs.com/docs/v21.2/cluster-settings.html this setting does not appear.

Jira Issue: DOC-3480