doc: clarified 'z-index' wording.
doc: clarified 'z-index' wording.

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