Update and rename 4 - Extending Controllers and Models with Decorators.t...

...extile to 4 - Extending Controllers with Decorators.textile

I removed all mention of extending models from this file (including the title) because there is another document on extending models and this document does not provide any direction for extending models.

Will need to be pulled into 2-0-stable as well.

With this addition the link from my previous pull request will probably break now that the name of this file is different so feel free to reject that pull request and I can fix it.