This rule is triggered when inconsistent styles of horizontal rules are used
in the document:

```markdown
---

- - -

***

* * *

****
```

To fix this, use the same horizontal rule everywhere:

```markdown
---

---
```

The configured style can ensure all horizontal rules use a specific string or it
can ensure all horizontal rules match the first horizontal rule (`consistent`).

Rationale: Consistent formatting makes it easier to understand a document.