Markdown formatter works only for errors - lack of documentation for formatters

[rartych]

Describe the bug
When there are no linting errors -f markdown produces only table header in Markdown.
For other formatters all errors (like stylish, github-action) are allways printed

To Reproduce

1. Given OpenAPI document without errors of severity error but with warning or info errors
2. run spectral lint with -f markdown option --> only table header in Markdown is printed
3. run spectral lint with -f markdown option and --fail-severity=info option --> Markdown table with all errors is printed

spectral lint --version
6.15.0

Expected behavior
Markdown table with all errors is printed without setting --fail-severity option

Additional context
More detailed documentation on available formats should be added to https://github.com/stoplightio/spectral/blob/develop/packages/formatters/README.md and https://docs.stoplight.io/docs/spectral/9ffa04e052cc1-spectral-cli#formatters