fixes bug 186513 "Auth prompting, etc. from incremental downloader" r=biesi a=bsmedberg

netwerk/base/public/nsIIncrementalDownload.idl

@@ -126,9 +126,12 @@ interface nsIIncrementalDownload : nsIRequest
    *        An observer to be notified of various events.  OnStartRequest is
    *        called when finalURI and totalSize have been determined or when an
    *        error occurs.  OnStopRequest is called when the file is completely
-   *        downloaded or when an error occurs.  If this object also implements
+   *        downloaded or when an error occurs.  If this object implements
    *        nsIProgressEventSink, then its OnProgress method will be called as
-   *        data is written to the destination file.
+   *        data is written to the destination file.  If this object implements
+   *        nsIInterfaceRequestor, then it will be assigned as the underlying
+   *        channel's notification callbacks, which allows it to provide a
+   *        nsIAuthPrompt implementation if needed by the channel, for example.
    * @param ctxt
    *        User defined object forwarded to the observer's methods.
    */