Docstrings

Summary:

• scalar: extend docstring of secp256k1_scalar_set_b32
• field: extend docstring of secp256k1_fe_normalize

This is a backport of secp256k1 PR713

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5388