Adding to the controllers as services cookbook

| Q                  | A
| ------------------- | ---
| Doc fix?        | yes
| New docs?    | no
| Applies to     | all

As per #2433, a the moment just some additional information. I will look at adding the pros and cons/reasons for doing this separately and also at interlinking to this page from more places.

I have just given one example of replacing a base controller helper method. Do we want to list them all as per my blog post or just leave it at that?