Note about access_token should be normative

[rhiaro]

In https://micropub.net/draft/#reserved-properties it says "MUST NOT store access_token" but that's in a Note which (I think) makes it non-normative. Sounds like it should be normative, maybe move it out of the Note? (or if it's not normative, drop the MUST)

[aaronpk]

It's meant to be normative, and I put it in the "note" thingy to emphasize it. Happy to move it out of that if putting it in the "note" block implies it's non-normative.