MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
@zdne Yes. I think they conceptually serve different purposes (one is like an abstract) the other like an intro. Regardless, I think it would complicate the spec to try and force the descriptions to be the same thing. Simpler as is in MSON. Trying to explain when multiline and when not and when is not worth the complexity that would be added to the spec, if I understand the question here correctly.