Update set.xml Specify the correct return type and values, CS

[mmalferov]

Let's tell the reader the truth :)

 * Adds a new element, or updates the value if the key already is part of the hash
 *
 * Parameters:
 * - hash: A valid quickhash
 * - key: The key
 * - value: The value
 *
 * Returns:
 * - 1 if the element is part of the hash and was updated, 2 if the element was
 *   added or 0 if an error occurred.
 */
int qhi_hash_set(qhi *hash, qhv key, qhv value)
{...

    if (find_bucket_from_list(...)) {
        return qhi_update_value_in_bucket(...); // returns 1
    } else {
        return qhi_add_entry_to_list(...) ? 2 : 0; // returns 2 or 0
    }...