[DOC beta] Clarify adapter settings with ActiveModel::Serializers

Clarify settings for using the DS.ActiveModelAdapter.

"You are intended to create an ApplicationSerializer and include `embed :ids, include: true` for Ember apps, which will make all your Rails serializers use sideloading."
   - See https://github.com/emberjs/data/pull/1615#issuecomment-37785301