[SECP256K1] Rename msg32 to msghash32 in ecdsa_sign/verify and add explanation

Summary:
Backport of secp256k1#854

Did the same for the schnorr module.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9376