fix docs for `path-add-suffix`
fix docs for `path-add-suffix`

Adjusts the docs, but a future replacement (possibly with a new name
to avoid compatibility issues) should behave like the old docs,
instead.

Closes #1292

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