haskell-process-path-* now allowed to be lists

[mikesperber]

The customize specs for haskell-process-path-ghci & friends say they can be lists of strings. But the code actually only works with strings.

(I want to use ("nix-shell" "--run" "ghci").)

[gracjan]

Thanks for the report. This should not be hard to fix. Can you help?

[mikesperber]

I'd be happy to - I assume you mean actually handling lists, right? (Rather than outlawing them at the defcustom.)

[gracjan]

Yes, handling lists.

[gracjan]

Hi @mikesperber, I noticed that docstrings for haskell-process-path-ghci and friend are confusing now. Can you help and write better description?

(While you are at it a section about those does not exist in doc/haskell-mode.texi. Can you add it there?)