AI 0028: New text for section 4 describing assertion/response version

Add to 4.1.3 or perhaps a new 4.1 subsection called "Permissible Version Combinations" or something similar:

"In general, assertions of a particular major version may appear in responses of the same major version, as expected from the
schema. Future versions of the specification are expected to explicitly describe the permitted combinations across major versions.

Specifically, this permits a 1.1 Assertion to appear in a 1.0 Response and a 1.0 Assertion to appear in a 1.1 Response."

-- Scott