Incorporate Commonalities WG recommendations on Simplification of Device object

[eric-murray]

What type of PR is this?

Add one of the following kinds:

• documentation

What this PR does / why we need it:

• Add Commonalities WG recommended text on "Identifying a device from the access token"
• Add 422 error response option
• Explicitly define request body as optional

Which issue(s) this PR fixes:

Fixes #61

Special notes for reviewers:

Request bodies were already optional

Changelog input

 release-note
 - Add Commonalities WG recommended text on "Identifying a device from the access token"
 - Add 422 error response option
 - Explicitly define request body as optional

Additional documentation

None