message: 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
diff: #include "scalar.h"
  #include "group.h"
  -/* Here `bits` should be set to the maximum bitlength of the _absolute value_ of `q`, plus
  - * one because we internally sometimes add 2 to the number during the WNAF conversion. */
  +/**
  + * Multiply: R = q*A (in constant-time)
  + * Here `bits` should be set to the maximum bitlength of the _absolute value_ of `q`, plus
  + * one because we internally sometimes add 2 to the number during the WNAF conversion.
  + */
  static void secp256k1_ecmult_const(secp256k1_gej *r, const secp256k1_ge *a, const secp256k1_scalar *q, int bits);
  #endif /* SECP256K1_ECMULT_CONST_H */
  