MSON AST reflecting the new specification

For @fosrias, @honzajavorek and @pksunkara to review. 
I would say I need to distinguish between inline and block descriptions in the AST in order to make this distinction clear, so I should rename it back to `block description`. 

Also note that, I would like to eventually support constructs like this: 

```
- property - inline description
    - Description

        Description 1

    - Properties
        - a
        - b
        - c

    - Description

        Description 2

    - Properties
        - d
        - e
        - f

    -  Description

        Description 3
```

And this is where you definitely do not want to concatenate descriptions strings on behalf of the AST user...