[Haddock] Issue with support for warnings/deprecations

[Simon Hengel]

> data Documentation =
>   {
>     doc :: Doc DocName;
>     warnings :: [Warning];
>     ...
>   }
> 
> and then store that in the ExportItems that we feed to the backends.

Yes, sounds good.  `Warning` would the be an other Doc node (or at least
contain one).  And for prune we'd only check, `doc`.

> > In addition I'd tend to implement #204 [1] before releasing that
> > feature.  Otherwise, it's not on par with handwritten deprecation
> > messages.
> 
> Ah, great!
> 
> Myself, I hope to have some time to work on Haddock this sunday.

Great, I'll probably be at MuHac, not sure how much I'll get done there.
Would be awesome if you can help (or at least give advise) with the
renaming of the warning Doc nodes.  I'm not very familiar with that part
of the code.

Cheers,
Simon