Updating documentation to warn users that { root: false } will ignore the meta key

I discussed this in https://github.com/rails-api/active_model_serializers/issues/332

I do not believe it is possible to include a meta key while using `{ root: false }`. This is fine - the JSON would be invalid... but it would have helped me to read it in the meta section of the documentation.

davingee was also confused by this, as seen in https://github.com/rails-api/active_model_serializers/issues/334