Merge bitcoin#634: Add a descriptive comment for secp256k1_ecmult_const.

ee9e68c Add a descriptive comment for secp256k1_ecmult_const. (Gregory Maxwell)

Pull request description:

  Helps issue bitcoin#633

ACKs for commit ee9e68:

Tree-SHA512: 552bebbd99bf8e8225ef6028e6a3bd188d412977d9c6caa90515041622accd2ea43e320217bf097180343921e967f4627a76c73e4529097bca50be414503e63b

src/ecmult_const.h

@@ -10,8 +10,11 @@
 #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 */