fix `impersonate-struct' docs
fix `impersonate-struct' docs

Move some constraints from `chaperone-struct' docs to `impersonate-struct'
docs.

(Problem reported by Eric Dobson.)

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