Improve comment for _fe_verify_magnitude

Author: peterdettman

src/field.h

@@ -126,7 +126,7 @@ static void secp256k1_fe_cmov(secp256k1_fe *r, const secp256k1_fe *a, int flag);
 
 #ifdef VERIFY
 
-/** Assert a lower limit m on the magnitude of a. */
+/** Assert a context-specific limit m on the magnitude of a. m cannot be outside the intrinsic range of magnitudes supported by this field implementation. */
 static void secp256k1_fe_verify_magnitude(const secp256k1_fe *a, int m);
 
 #endif