Remove outdated model documentation

Since the static `find` method on models was [removed over a year ago](https://github.com/emberjs/data/commit/d96327c30a74a55aa75ed42b73b95198e5f9f024#diff-4cc9f4885e8425eeaa555ef3648588ae), I removed some of the references to it and used the store's `find` method instead.