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

9b9046e0482ed5858c71fdda17d20dce1c0a1b57 helped, but was insufficient. Anecdotal evidence of that insufficiency below.

/cc @jessebeach

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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...

From http://stackoverflow.com/questions/16823746/backbone-js-listento-vs-on:

> If you provide an explicit context (the 3rd argument to `foo.on` or the 4th argument to `this.listenTo`), backbone will use that (thus this is a more robust approach)

(Now fixed since I pointed out that problem: http://stackoverflow.com/questions/16823746/backbone-js-listento-vs-on#comment31091515_16824080 .)