clarify support and behavior around embedded records mixin

[ghost]

Initial documentation was landed in #6958, but we should make it clearer for folks by default. After discussing in the ember-data core meeting on 2020-02-19, we (@hjdivad, @rwjblue, and @efx) propose:

Updating the embedded records mixin to assert when used with JSONAPISerializer unless a specific flag to opt out of the assertion is used.

Something like:

assert(
  'The embedded records mixin does not work with all JSON:API\'s, please confirm this works for your specific API and add `this.isEmbeddedRecordsMixinCompatible = true` to your serializer', 
  this.isJSONAPISerializer !== true || this.isEmbeddedRecordsMixinCompatible === true
);

[adriancerejo]

I am fairly new to open source contributions and was wondering if I could take this issue.

[allthesignals]

Hmm I think it might go here: https://github.com/emberjs/data/blob/v3.17.0/packages/serializer/addon/json-api.js

I'm not sure where makes the most sense, maybe the constructor?

EDIT: Oh, maybe here: https://github.com/emberjs/data/blob/v3.17.0/packages/serializer/addon/json-api.js#L560-L590

The original warning could be turned into an assert?

[hjdivad]

@allthesignals @adriancerejo changing the existing warning you identified to an assertion with an opt-out flag would be great.

[allthesignals]

@hjdivad I've got a branch but I'm not sure which to PR into, gonna check docs in a sec :)