add REST documentation for mediacore module

This is a bit weird. `media_id` is being passed in as part of the POST body. It doesn't make a lot of sense that we then have to identify `body` as the `BodyParam`. `BodyParam` already specifies that we're in the POST body. 

The Swagger UI also seems to indicate that something isn't right. It just offers me a textarea for `body`, which is rather unhelpfel, rather than the expected input field for `media_id`