Clarify how the name of the prefetch key should be used in the context of evaluating a single CQL expression (if it should be used at all).

For example, the description for prefetchData.key states:

This typically corresponds to the name of a parameter in a library, or the name of a prefetch item in a CDS Hooks discovery response

If the prefetch key does match a parameter name, should it be supplied as a parameter value to the expression? What if it does not match a parameter name? Should the CQL expression still be able to reference it by name (as if it was defined as a statement within a CQL library)?

This clarification is important to ensure consistent behavior across implementations.