add REST documentation for content module

The code is still in master:

```
/*!
 * Join a collaborative document
 */
OAE.tenantRouter.on('post', '/api/content/:contentId/join', function(req, res) {
    ContentAPI.joinCollabDoc(req.ctx, req.params.contentId, function(err, data) {
        if (err) {
            return res.send(err.code, err.msg);
        }
        res.send(200, data);
    });
});
```