[SECP256K1] Consistency improvements to the comments
[SECP256K1] Consistency improvements to the comments

Summary:
Completes 11/11 backport of secp256k1 [[https://github.com/bitcoin-core/secp256k1/pull/830 | 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

Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc