Note intention of timing sidechannel freeness.
Note intention of timing sidechannel freeness.

Summary:
Resolves #238

This is a backport of secp256k1 PR630

Test Plan: \o/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5000

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