Add JSDoc to marionette.helpers

Used a bunch of descriptions from https://github.com/Puppets/marionette-docs/blob/master/api/helpers/helpers.md
This is explicitly mentioned in our docs as a feature for the end user: https://github.com/marionettejs/backbone.marionette/blob/master/docs/marionette.functions.md#marionetteactascollection

If we're not including it in the jsdoc, we should also probably remove it from the existing docs as part of the public api.