[Common UI] Add note about TODO item

Add note about temporary work around, per code review
from nasa/openmctweb#299

platform/commonUI/general/src/directives/MCTPopup.js

@@ -51,6 +51,8 @@ define(
                     position = [ rect.left, rect.top ],
                     popup = popupService.display(div, position);
 
+                // TODO: Handle in CSS;
+                //       https://github.com/nasa/openmctweb/issues/298
                 div.css('z-index', 75);
 
                 transclude(function (clone) {