Section 3.8 - Prohibit type annotations with type unions

[clemensv]

Add a clause that explicitly prohibits using type annotations in schemas where the type is a type-union.

This prevents ambiguous definitions where the type union includes a combination of types (e.g. ["string", "decimal"]) and the annotation would only apply to one of the types.

To achieve a type union of further constrained types, one must define them as reusable types and type-union those.

The prohibition will also extend to and be repeated in the validation spec.