Update batch delete guidance for performance improvements

Eric Harmeling (ericharmeling) commented:

this example will cause performance issues in production based on my testing of what [this webpage](https://www.cockroachlabs.com/docs/stable/bulk-delete-data.html) recommended, and to avoid that, one should do

```
BEGIN
SELECT FROM rides AS OF SYSTEM TIME -30s WHERE
ROLLBACK

BEGIN LOW PRIORITY TRANSACTION; DELETE FROM rides WHERE PK IN (...); COMMIT
```

_Originally posted by @otan in https://github.com/cockroachdb/docs/pull/12114#discussion_r741472484_

Jira Issue: DOC-1137