Add a descriptive comment for secp256k1_ecmult_const.
Add a descriptive comment for secp256k1_ecmult_const.

Summary: This is a backport of libsecp256k1 PR634

Test Plan:
  ninja all check-secp26k1

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D5349

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