doc: clarify input requirements for secp256k1_fe_mul

"... neither can be equal to b." could suggest that the values are not
allowed to be identical, but what is meant here is that the mentioned
inputs shouldn't point to the same object.

src/field.h

@@ -255,8 +255,8 @@ static void secp256k1_fe_add(secp256k1_fe *r, const secp256k1_fe *a);
 /** Multiply two field elements.
  *
  * On input, a and b must be valid field elements; r does not need to be initialized.
- * r and a may point to the same object, but neither can be equal to b. The magnitudes
- * of a and b must not exceed 8.
+ * r and a may point to the same object, but neither may point to the object pointed
+ * to by b. The magnitudes of a and b must not exceed 8.
  * Performs {r = a * b}
  * On output, r will have magnitude 1, but won't be normalized.
  */