[SECP256K1] Consistency improvements to the comments
Summary:
Completes 11/11 backport of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/c582abade1c50ef50dc7ee9f7b7af8e06e22065d
Depends on D8046.
Test Plan: ninja check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8047