Document background reloading options for Ember Data

[acorncom]

See https://github.com/emberjs/data/issues/4720 These features were introduced in Ember Data 1.13 (see the blog post about it: https://emberjs.com/blog/2015/06/18/ember-data-1-13-released.html) but never written up for the guides. We don't necessarily want a lot of information about it (it'd be better to send folks to the API docs) but it should at least be mentioned so folks know the concept exists ...

[RyanNerd]

@acorncom I'll see if I can create a PR for this over the weekend.

[runspired]

Seems like this is still needed.