spec: for struct types, clarify field "name" and "scope" of field names/selectors

[griesemer]

Field names are identifiers "declared" with a struct type definition. The section of struct types could be clearer with respect to what a "field name" is. E.g.: struct{T} vs struct {T T} vs struct {t T} .

Also, the "scope" of field names/selectors is not explicitly defined. Determine if there's any clarification needed.