Add a section on Questionnaire and QuestionnaireResponse (in the core spec) that explains when it's appropriate to profile Questionnaire vs. creating an instance and when to profile QuestionnaireResponse as opposed to defining a Questionnaire.  Specifically, profiles on Questionnaire and QuestionnaireResponse are appropriate to define general capabilities or restrictions on a 'set' of Questionnaires (e.g. This system doesn't support answers of type 'Reference').  Useful for defining the capabilities of a particular application or ecosystem.