add REST documentation for content module

Could I be looking in the wrong place. I see exactly those three

```
OAE.tenantRouter.on('get', '/api/content/:contentId/revisions/:revisionId/previews/:item', function(req, res) {
    var signature = {
        'signature': req.query.signature,
        'expires': req.query.expires,
        'lastModified': req.query.lastmodified
    };
```