Add JSDoc to marionette.helpers

Used a bunch of descriptions from https://github.com/Puppets/marionette-docs/blob/master/api/helpers/helpers.md
Interesting, I don't believe that this shouldn't be documented. This is much more than a private function...it's a completely inaccessible function from the outside!

If we want to document it, then we will also need to attach it to Marionette.