attempt to clarify the overlay/xy and underlay/xy documentation   Merge to 5.1
attempt to clarify the overlay/xy and underlay/xy documentation
  Merge to 5.1

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