A new trait to retrieve doc comments (if available).

[bearophile via Digitalmars-d]

Mason McGill:

> Other implementations can choose to always evaluate it to "".

Other implementations have to give the ddostring as well.

In D modules too have a ddoc string.

Regarding comments on single variables, like this, I think they 
can be ignored for the moment, and added later with the same API 
if needed:

int foo = 5; /// Not a string.

Bye,
bearophile