The current description says:
"For items of type reference, the reference can be populated either by picking an existing entity or by creating a new one. In the latter case, this Questionnaire allows specifying the Questionnaire to use in creating the target instance.This can be direct (if the Reference is to a Questionnaire) or indirect via mapping if the Reference is to some other sort of resource."

The "direct" case (where the Reference is of type Questionnaire) needs clarification, or perhaps removal. It seems to me that if the question's answer is a reference to a Questionnaire, and the user is responding by creating a new Questionnaire (using the lookupQuestionnaire to do it), you would still need the mapping just as you would for any other resource type.