Document the additionalData argument

[isometriks]

https://github.com/less/less.js/blob/master/lib/less/parser.js#L361

• globalVars
• modifyVars
• banner

Are these things planning on staying this way, or are they not intended to be documented as they might change? I see that globalVars is documented with the browser version, but really it just sets one of these additionalData variables to be used in the parse method.