Allow users to add commentary to Haddock docs

[simonpj]

It would be great if Haddock generated web pages that allowed users to add commentary to the documentation. That would make it much more interactive. The PHP manual does this, to good effect  http://blog.milkfarmsoft.com/?p=29

In fact, Haddock already supports something like this. It has a flag that adds a Wiki link to every function definition; users can click on the link to get to a Wiki page that they can then add material to. But it's not perfect:

• Most of the Wiki links will be empty, but there's no way to tell that when looking at the link. It'd be better to highlight populated links somehow.
• The library author may want to look at all the comments for a module, with a view to enhancing the Haddock docs; but it would be unbearably tedious to go through them all one by one.
• There is no place for overall comments

There is real potential here, but it needs someone to think about it, and implement something a bit better.

[igloo]

I've added the above to the haddock ticket on the same subject:  http://trac.haskell.org/haddock/ticket/72#comment:5