Update documentation.md

I've been confused by inconsistent behaviour comparing to native `<input type='file' multiple>` in `uploadMultiple: true` mode. Dropzone strangely adds postfixes to each file in request, so that it looks like `files[0], files[1], ...`, whereas native input's behaviour is just `file, file, ...`. To cover native behaviour it's not enough to set `paramName: 'file'`, you rather need to define a function: `paramName: function(){return 'file'}`, but that isn't clear from the docs.

I think that maybe it is even worth noting separately somewhere in the docs, but at least you have to have it in API description.