Add details on connecting to a virtual cluster using native cockroach sql options

Regarding this page: https://www.cockroachlabs.com/docs/stable/work-with-virtual-clusters

It describes how to connect to a virtual cluster by using a long-winded postgresql URL (`postgresql://....`. While this is fine, there's also the option to use the cockroach sql client like this:

```
cockroach sql --database cluster:/
```

Sometimes, passing the cluster name in like this is easier than construction a long postgresql URL.

Could we add the use of `cockroach sql --database` to this page?

Jira issue: DOC-16297