Make docs better. Bug 345069, r=pavlov

modules/libpr0n/public/imgILoader.idl

@@ -93,7 +93,9 @@ interface imgILoader : nsISupports
 
   /**
    * Start the load and decode of an image.
-   * @param uri the URI to load
+   * @param aChannel the channel to load the image from.  This must
+   *                 already be opened before ths method is called, and there
+   *                 must have been no OnDataAvailable calls for it yet.   
    * @param aObserver the observer
    * @param cx some random data
    * @param aListener [out]