Better error for level 1 headings

```R
Warning: [across.R:42] @returns markdown translation
failed
✖ level 1 markdown headings are not currently
  supported
```

Should be:

```R
Warning: [across.R:42] @returns markdown translation
failed
✖ level 1 markdown headings are not currently supported in `@returns`
? Do you want to put the block after the description?
```
