add REST documentation for mediacore module

Just confirmed that pretty much all of the POST requests we do from the UI are done as `application/x-www-form-urlencoded`. That means we should be using `form` for all of those, right? If that's the case, we should figure out how to do optional form parameters.

@sathomas : I also seem to remember that you said that MediaCore sends this particular request as `application/json`. How did you verify this?