Update documentation.md

I've been confused by inconsistent behaviour comparing to native `input multiple`. Dropzone in `uploadMultiple` mode 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, ...`. So to cover native input behaviour it's not enough to point in options `paramName: 'file'`, you rather need to define a function: `paramName: function(){return 'file'}`, and that isn't clear from the docs.

I think that it is maybe worth noting separately somewhere in the docs, but at least you have to have it in API docs.
Merge pull request #715 from dfcreative/patch-1

Update documentation.md