220111 nsIDownload should document that Init must be called

r=bzbarsky sr=darin

uriloader/base/nsIDownload.idl

@@ -49,7 +49,8 @@ interface nsIMIMEInfo;
 interface nsIDownload : nsISupports {
     
     /**
-     * Initializes the download with certain properties.
+     * Initializes the download with certain properties.  This function must
+     * be called prior to accessing any properties on this interface.
      *
      * @param aSource The source (nsIURI) of the download.
      *