How to document parameters of a closure that's a parameter

Question

My attempt at documenting a closure parameter to a function:

/**
    sends the request out.

    - Parameter pzurl: request description including url, method, formdata.
 - Parameter responseProcessor: closure that's on success invokes
 - Parameter json: a json array or dictionary
 - Parameter timestamp: and the same `timestamp` that this function returns
 - parameter failure: closure is invoked when json could not be parsed or remote indicated failure
 - Returns: timestamp to identify responses
 */
internal static func processPZURL(pzurl: PZURL,
                                  responseProcessor:@escaping (_ json:JSON, _ timestamp: TimeInterval) -> (), failure:@escaping (PZError) -> ()) -> TimeInterval {

yields "No description"

and I can find no description of how to change that