Clarify that listenTo() does not allow a context to be supplied

9b9046e0482ed5858c71fdda17d20dce1c0a1b57 helped, but was insufficient.

From https://drupal.org/comment/8299301#comment-8299301:

>>  [listenTo](http://backbonejs.org/#Events-listenTo): Tell an object to listen to a particular event on an other object...The callback will always be called with object as context.
>
> Honestly, that quote from the docs is....damn muddled. Which object now? The object invoking `listenTo` or the one raising the event? Let's check the code in Backbone...

/cc @jessebeach