Survey profile Response entities should be more explicit about their contained value types

[ViktorHaag]

Right now, the Response entities in the Survey profile document all carry a "response value" that is associated with the expected kind of value you'd get for a response based on the Question type associated with the Response.

The Response types carrying a single "value" field do explicitly express the type of value they contain, but the ones with a "selections" fields do not:

• MultiSelectResponse selections items should be strings
• RatingScaleResponse selections items should be strings or decimals based on the underlying Scale backing the question associated with the response