Document that a FinalizerPtr is a pointer to a ccall function (#5254)

It can't be any other calling convention, e.g. stdcall.

libraries/base/GHC/ForeignPtr.hs

@@ -114,6 +114,8 @@ instance Show (ForeignPtr a) where
 -- finalisation time, gets as an argument a plain pointer variant of the
 -- foreign pointer that the finalizer is associated with.
 -- 
+-- Note that the foreign function /must/ use the @ccall@ calling convention.
+--
 type FinalizerPtr a        = FunPtr (Ptr a -> IO ())
 type FinalizerEnvPtr env a = FunPtr (Ptr env -> Ptr a -> IO ())