code: public static void addPrefsDirective(Element plfNode, String attributeName, IPerson person)
              throws PortalException {
          addDirective(plfNode, attributeName, Constants.ELM_PREF, person);
      }
docs: Create and append a user preferences edit directive to the edit set if not there. This only
  records that the attribute was changed. The value will be in the user preferences object for
  the user.