Tweaked a comment to make attribute behaviour more clear. b=131725

docshell/base/nsIWebNavigation.idl

@@ -187,7 +187,7 @@ interface nsIWebNavigation : nsISupports
   readonly attribute nsIDOMDocument document;
 
  /**
-  * The current URI that is loaded.
+  * The currently loaded URI or null.
   */
   readonly attribute nsIURI currentURI;