add REST documentation for content module

Now I can't figure out where I got those properties. Tracing the code, I reach a bit of a wall in `internal/dao.previews.js`:

```
var contentUpdate = _.extend({}, contentMetadata, {'previews': previews});
queries.push(Cassandra.constructUpsertCQL('Content', 'contentId', contentObj.id, contentUpdate));
```

It sort of looks like you could have any property here and it would get updated, but I don't know what goes on within cassandra. Would the preview processor actually even want to change `description` or `displayName`? It kind of seems like this whole object shouldn't be there, but the code will error out if it's not present.