ffi/unsafe docs: improve `_cprocedure' docs
ffi/unsafe docs: improve `_cprocedure' docs

Among other improvements, add information about reachability of
callout arguments.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket