Docs: add notificationBanner to feature toggle list

Patrick Leong has reported that a user pointed out in a [recent issue](https://grafana.zendesk.com/agent/tickets/214116) that the `notificationBanner` feature toggle is not on the [list of feature toggles](https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/feature-toggles/). He also asks that we make sure to mention that it is not available on OSS.