user-guide: Note Cabal version limitation
user-guide: Note Cabal version limitation

(cherry picked from commit b90cac69b2abec7df998d1d447b3fe075eb8f15c)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc