Updating run loop documentation

Backburner was updated and the documentation for
throttle/debounce/cancel needs to be updated to reflect
new functionality

Previous PRs: https://github.com/emberjs/ember.js/pull/3886 and https://github.com/ebryn/backburner.js/commit/5c948e6d809441315f06c5d1787fca31b509409a