Summary:
The generated documentation for thhe Foreign.C.Types
module is generated based on the platform which ran Haddock.

This is generating incorrect types for e.g. Windows.

Add a disclaimer to the top of the page to ask people to
keep this in mind.

Test Plan: make documentation and inspect Haddock

Reviewers: erikd, austin, hvr, bgamari

Reviewed By: erikd

Subscribers: RyanGlScott, #ghc_windows_task_force, thomie

Differential Revision: https://phabricator.haskell.org/D2523

GHC Trac Issues: #12586