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 Work for 3.10 or 3.11 - update code blocks to reflect new import paths for ember-data packages #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 emberjs/rfc-tracking#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.