Document access token format (is a JWT itself)

It seems that the access token being issued by https://www.adobe.io/authentication/auth-methods.html#!AdobeDocs/adobeio-auth/master/JWT/JWT.md#exchanging-jwt-to-retrieve-an-access-token is a JWT itself but it doesn't seem to follow https://tools.ietf.org/html/draft-ietf-oauth-access-token-jwt-07#section-2.

By looking at the AEM client (com.adobe.granite.auth.ims.impl.IMSTokenValidatorImpl) I assume that at least the following claims are contained in there

1. created_at
2. expires_in

Please document the format of the access token so that it can be validated without having to consider the other properties from the OAuth response (https://tools.ietf.org/html/rfc6749#section-4.1.4).

Start with the linked Adobe JWT authentication documentation and OAuth sections, then compare the described token with AEM's IMSTokenValidatorImpl reference. Document the access-token format and claims, including created_at and expires_in if confirmed, so it can be validated without relying on other OAuth response properties.

Written by the indexing model from the issue text.