artart review notes

[aaronpk]

Summarized action items based on review:

https://mailarchive.ietf.org/arch/msg/oauth/lyq7LLM5AFLayFsjG_bg4g6k0bs/

• expand PKCE reference in Section 4

[aaronpk]

Responses posted to the list:

https://mailarchive.ietf.org/arch/msg/oauth/p_JzBshJQbbzraZH7xabBwZ44x0/