[docs] [bugfix] for `itemView` option of `each` helper 

In  ember-cli world where we encourage people to register all views in container, it is nice to provide `each` helper item view with `itemView` syntax, not with `itemViewClass`. Having this info in docs should speed up adoption of new pattern. 

By this occasion a bugfix was applied to disable `group` mode if `itemView` option was specified.