add comment for append/prepend z-indexing

less/forms.less

@@ -368,6 +368,7 @@ select:focus:required:invalid {
     *margin-left: 0;
     vertical-align: middle;
     .border-radius(0 3px 3px 0);
+    // Make input on top when focused so blue border and shadow always show
     &:focus {
       position: relative;
       z-index: 2;