Add javaDoc for MultipartCallback

Author: nitzanj

cloudinary-android/src/main/java/com/cloudinary/android/MultipartUtility.java

@@ -141,6 +141,9 @@ public HttpURLConnection execute() throws IOException {
         return httpConn;
     }
 
+    /***
+     * For internal use only - callback to monitor multipart upload progress
+     */
     interface MultipartCallback {
         void totalBytesLoaded(long bytes);
     }