Clearer description of the binder notation `{T=U}`

The last part of the [Named Type Parameters](https://github.com/fram-lang/fram-doc/blob/master/src/intro/named-parameters.md#named-type-parameters) section, which introduces binders of the form `{T=U}`, has been identified as a bit hard to understand to newcomers. Perhaps it could be improved by a comment in code pointing out the `T` defined above in the annotation for `x` in `foo`?