Work for 3.10 or 3.11 - update "mental model" of ember-data

[jenweber]

This ember-data RFC proposes some improvements to the mental model of ember-data. We should update the guides to reflect it. Your PR should branch from and target master.

To do

• [ ] Have a read through the RFC
• [ ] Read through the Ember Data Guides
• [ ] Make a list of written sections that you think should be changed, and post it in this issue (ignore changes to code blocks, which will be done separately in #740 )
• [ ] Either edit the sections to reflect the new mental models, or open issues for each section that should update.
• [ ] Open small PRs, one for each page of the guides that you are changing

Depending on the edits, these PRs may be merged right away, or when ember-data's package refactor lands in a stable release (probably 3.10 or 3.11)

Part of work for https://github.com/emberjs/rfc-tracking/issues/11

[runspired]

@efx is spearheading an effort to audit the docs in ember-data over the next 6 months, would be a good time for some learning team members to do the same for guides.