No customize option for haskell-mode-hook

In the Haskell customize group there is only:
- Haskell Completing Read Function
- Haskell Language Extensions
- Haskell Stylish On Save
- W3m Haddock Heading Face
and the Haskell Interactive Settings.

I checked some other modes and they all have a foo-mode-hook customize option. Also, the documentation recommends `M-x customize-variable RET haskell-mode-hook`, but this returns '[No match]'

Search for the haskell-mode-hook definition and the Haskell customize group, then compare them with the documentation's customize-variable command. Confirm that the hook appears in the Haskell customize group and that customizing haskell-mode-hook no longer returns '[No match]'.

Written by the indexing model from the issue text.