Add optional documentation to declaration items in (decl) form.

[kiselgra]

I only added support for this for cm-c and cm-cxx. If you feel that the changes are alright, then I'll fix the others.

I mainly want to get your input on the pretty printer code. The outline is:

• While writing the subnodes of declaration-items, the traverser will not emit comments
• When the declaration-item is done and has a comment field, the comment is emitted after the ;

My main question is: is there a better way to do this? because getting the comment node itself and accessing the pretty printer was quite fiddely.