Add JSDoc for `FormView.prototype.pages.next()`
Add JSDoc for `FormView.prototype.pages.next()`

Issue: #339

enketo/enketo-core,medic/enketo-core,nap2000/enketo-core,nap2000/enketo-core,enketo/enketo-core,nap2000/enketo-core,kobotoolbox/enketo-core,enketo/enketo-core,kobotoolbox/enketo-core,medic/enketo-core