Clarify that a secp256k1_ecdh_hash_function must return 0 or 1

Summary:
and improve style of the ECDH docs.

This is a backport of libsecp256k1 PR718

Depends on D5389

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5390