MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
I understand, but what if a property have both `inline description` and `block description`? The code working with the MSON AST would have to check for descriptions in two different **kind** of places.

I am just saying that, it's better if we add another section `inline description` and move this `description` to `sections` or just convert every `inline description` to `block description` in `sections` (similar to how we did in Blueprint AST).