Documented the security:check command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4051
This won't work given that `/book/security/checking-vulnerabilities.rst` is no document. But `book-security-checking-vulnerabilities` is a label and you can use the `ref` directive instead:

```rst
[...] command (see :ref:`book-security-checking-vulnerabilities`).
```