doc: make clear that the arguments are never null
doc: make clear that the arguments are never null

dminkovsky/graphql-java,blevandowski-lv/graphql-java,fderose/graphql-java,graphql-java/graphql-java,fderose/graphql-java,blevandowski-lv/graphql-java,dminkovsky/graphql-java,graphql-java/graphql-java