field.h: improve doc of fe_equal_var

- `_fe_equal` requires both the inputs magnitude to be 1
- `_fe_equal_var` requires only the first input magnitude to be 1

src/field.h

@@ -64,10 +64,13 @@ static int secp256k1_fe_is_zero(const secp256k1_fe *a);
 /** Check the "oddness" of a field element. Requires the input to be normalized. */
 static int secp256k1_fe_is_odd(const secp256k1_fe *a);
 
-/** Compare two field elements. Requires magnitude-1 inputs. */
+/** Compare two field elements. Requires both the inputs to have magnitude = 1.
+ *  i.e. a->magnitude = 1 and b->magnitude = 1 */
 static int secp256k1_fe_equal(const secp256k1_fe *a, const secp256k1_fe *b);
 
-/** Same as secp256k1_fe_equal, but may be variable time. */
+/** Same as secp256k1_fe_equal, but may be variable time.
+ *  Requires only the first input to have magnitude = 1. 
+ *  i.e. a->magnitude = 1 */
 static int secp256k1_fe_equal_var(const secp256k1_fe *a, const secp256k1_fe *b);
 
 /** Compare two field elements. Requires both inputs to be normalized */