racket/gui: fix docs on reparenting (which is now possible)
racket/gui: fix docs on reparenting (which is now possible)

Closes PR 13108

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