user-guide: Various fixes to FFI section
user-guide: Various fixes to FFI section

shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc