[ci skip] Fix doc usage of :source_path
[ci skip] Fix doc usage of :source_path

A follow up from https://github.com/ai/autoprefixer-rails/pull/89 we were accidentally recommending to use the wrong API when building your own processor. 

I also added docs for all the different keys that we pass to processors for future reference.
prathamesh-sonpatki/sprockets,eileencodes/sprockets,rails/sprockets,recursecenter/sprockets