user-guide: Add cross-reference for -XUnicodeSyntax
user-guide: Add cross-reference for -XUnicodeSyntax

(cherry picked from commit 6cebfdb44c9d2f893a5049b44489015fd5381342)

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