Amend RFC 0821 given implementation tweaks

[chriskrycho]

trafficstars

Authoring the types for DefinitelyTyped revealed a number of issues, which were also surfaced previously while working on Ember's internal versions of these types. Accordingly, align the types as defined here with Ember's internal representation of them, and clarify why the types are the way they are when they may not seem especially sensical. Also add a FullName export for convenience.

As a bonus, add a table of contents and make sure the order of the sections matches the order described in prose.

[chriskrycho]

We are all agreed this is 👍🏼 and it’s going into FCP-to-merge.

[chriskrycho]

Looks good, merging!