Use `model` instead of `content` when dealing with Controllers.

This is mostly additional cleanup to ensure that both the docs and tests
reflect the change made in https://github.com/emberjs/ember.js/pull/4725
and https://github.com/emberjs/ember.js/pull/4857.
Merge pull request #4896 from rjackson/cleanup-references-to-content

Cleanup docs & tests to use `model` instead of `content` when dealing with Controllers.