Document 1.5 [ember-eager-update-url] changes.

One sentence to describe a behavior that surprised me since I didn't notice ember-eager-update-url as it landed.
https://github.com/emberjs/ember.js/pull/4122
https://github.com/emberjs/ember.js/issues/4574
Merge pull request #1395 from nathanhammond/patch-2

Document 1.5 [ember-eager-update-url] changes.