Documentation of RuntimeRep is wrong about 64-bit integer reps

Documentation of Int64Rep and Word64Rep currently have the note "on 32-bit only" which seems wrong. Here's an example (this is on a 64-bit system):

~ $ cat Main.hs
{-# LANGUAGE TypeApplications, MagicHash #-}

import Type.Reflection
import GHC.Types
import GHC.Prim
import Data.Int

main = do
  print (splitApps (typeRepKind (typeRep @Int64#)))
  print (splitApps (typeRepKind (typeRep @Int#)))

~ $ ghc Main.hs
[1 of 1] Compiling Main             ( Main.hs, Main.o )
Linking Main ...

~ $ ./Main
(TYPE,['Int64Rep])
(TYPE,['IntRep])

The documentation seems to suggest that Int64Rep should only be used on 32-bit systems, and we should get IntRep for Int64# on 64-bit.

(Link to RuntimeRep)

(The same comments exist in GHC HEAD source tree too)

(cc @rae)