add REST documentation for content module

It's just a renaming suggestion. Even though it departs a little bit from the API models, I think it would be better to have `BasicContent` and `Content`. Things like `Content` and `FullContent` would be fine as well.

The reason why I don't think `ContentProfile` is appropriate is because it would mean that we have to define `UserProfile`, `GroupProfile`, `DiscussionProfile`, etc. for the other full objects as well. However, we are close to introducing explicit user and group profile functionality, and at that point having `UserProfile` and `GroupProfile` as models for something else might become too confusing.