[SECP256K1] Detailed comments for secp256k1_scalar_split_lambda

Summary:
Partial backport 3/11 of secp256k1 PR830:
https://github.com/bitcoin-core/secp256k1/pull/830/commits/acab934d24ff26289ab9930587c3fc51c30c6a2f

Depends on D8038.

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8039