add REST documentation for mediacore module

This is either not correct or the documentation in `api.js` is out of date:

```
 * @param  {Object}     callback.embedCode          A JSON object with one element, 'html', which is the html code that should be inserted in the page
 * @param  {String}     callback.embedCode.html     The HTML source that can be used to embed this content item's MediaCore video in a page
```