doc: refined z index description. references #4252
doc: refined z index description. references #4252

@dessant. Indeed, a clearer way to state it. Should we also add a
explanation about widgets added in kv? Did not because I think it's very
intuitive?

inclement/kivy,janssen/kivy,aron-bordin/kivy,autosportlabs/kivy,darkopevec/kivy,Cheaterman/kivy,inclement/kivy,janssen/kivy,LogicalDash/kivy,kivy/kivy,matham/kivy,jegger/kivy,jegger/kivy,akshayaurora/kivy,manthansharma/kivy,LogicalDash/kivy,matham/kivy,autosportlabs/kivy,matham/kivy,LogicalDash/kivy,aron-bordin/kivy,rafalo1333/kivy,tony/kivy,akshayaurora/kivy,jkankiewicz/kivy,jegger/kivy,kived/kivy,KeyWeeUsr/kivy,tony/kivy,Cheaterman/kivy,bionoid/kivy,rnixx/kivy,inclement/kivy,darkopevec/kivy,rafalo1333/kivy,darkopevec/kivy,bionoid/kivy,KeyWeeUsr/kivy,rafalo1333/kivy,rnixx/kivy,Cheaterman/kivy,tony/kivy,manthansharma/kivy,janssen/kivy,janssen/kivy,bionoid/kivy,jkankiewicz/kivy,manthansharma/kivy,jkankiewicz/kivy,Cheaterman/kivy,matham/kivy,manthansharma/kivy,LogicalDash/kivy,autosportlabs/kivy,aron-bordin/kivy,KeyWeeUsr/kivy,aron-bordin/kivy,kivy/kivy,bionoid/kivy,jkankiewicz/kivy,kived/kivy,kivy/kivy,darkopevec/kivy,akshayaurora/kivy,kived/kivy,KeyWeeUsr/kivy,jegger/kivy,rnixx/kivy