Document that default-language defaults to Haskell98

I believe in the last Cabal dev meeting we agreed that it would be better for Cabal to default to `Haskell98` when no `default-language` is specified in the package. This is already the current behaviour, however, I believe it is undocumented.

This ticket tracks documenting that not specifying a default-language makes Cabal default to `Haskell98`