Signals that this [DownloadHandler] is no longer being used, and should clean up any resources it is using.
This method must be called once you have finished using a [DownloadHandler] object.
For convenience, [UnityWebRequest] exposes the [UnityWebRequest.disposeDownloadHandlerOnDispose] property, which will automatically call this method when [UnityWebRequest.Dispose] is called.
If you elect not to use [UnityWebRequest.disposeDownloadHandlerOnDispose] (by setting it to false), then you must call Dispose on the [DownloadHandler] yourself. This should only be in rare cases, such as when you wish to save the data in the [DownloadHandler]'s internal buffer(s) for later processing.