Align UAHF spec with upstream Bitcoin-UAHF

Summary:
This updates two definitions recently improved
on the upstream spec document.

Test Plan:
read / compare with upstream version at
https://github.com/Bitcoin-UAHF/spec/blob/master/uahf-technical-spec.md

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan, awemany, Mengerian

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D289