explain skipped test

Author: dfreedm

test/unit/dir.html

@@ -142,6 +142,10 @@
       });
 
       test('elements with dir attribute explicitly set will not change', function() {
+        /*
+        Both the polyfill and native ShadowDOM transforms to the `:dir` selector do not support
+        per-instance opt-out at this time.
+        */
         this.skip();
         var inner = fixture('preset');
         var innerRoot = Polymer.dom(inner.root);