prevent confusion about args of input function (e2e testing)

Updating docs to prevent confusion about args of input() function being an ng-model vs HTML "name" attribute: http://docs.angularjs.org/guide/dev_guide.e2e-testing#comment-941247365, http://docs.angularjs.org/guide/dev_guide.e2e-testing#comment-734673956, http://docs.angularjs.org/guide/dev_guide.e2e-testing#comment-898079915