Add inline documentation for scss functions

- Sticking to JSDoc conventions as closely as possible
- Using $ for @param names
- Using @warn instead of @throws
- Using {list} type for Scss lists
Add inline documentation to stylus functions

It's ported over from the scss version, but with
any irrelevant bits like non existant parameters removed.
Add inline documentation to the Scss mixin set
Add inline documentation to the Stylus mixin set