Use MSON syntax for description

This PR is for @honzajavorek to read & merge 
FWIW, the "appendix" is usually referenced to as a "suffix". I'd vote for sticking to terms one can find in various [specs](http://tools.ietf.org/html/rfc6839) and other [tools](https://github.com/expressjs/media-typer#typerparsestring).