The command cabal haddock has very useful --hyperlink-source option.
I would like to have the source hyperlinked when building documentation with cabal in
Ticket #517 is about passing haddock options via cabal, like if you have special needs like building a hoogle database or usa a different CSS
Most users will be happy with standard haddock and hyperlinked sources though, which is possible with Cabal-1.8. Ticket #517 is if you have bigger needs.
Simply calling cabal haddock --hyperlink-source should do the trick.
If you want this as the default, you can edit ~/.cabal/config as you mentioned.