Clarify: Specifying an unbound function with overloads in a function import should import all overloads of that function to the entity container

It's not clear from the spec, if a function import refers to a function with multiple overloads, which overloads should be imported into the entity container. The intention (which would reasonable to interpret from the spec) is that all unbound overloads are imported, but spelling that out in the spec makes it clearer for implementers.