Data.List documentation improvements

[Simon Peyton-Jones]

| I think what is really needed is for Haddock to have some kind of
| "show source" link that shows the source for a function inline in the
| documentation without opening a new page. Often the best documentation
| for a function is its definition.

I believe that Simon M has already implemented exactly that.  I'm not sure whether it takes extra flags or what.

Simon