update xbps_transaction_update_packages(3) doc

Author: fvalasiad

include/xbps.h.in

@@ -1234,6 +1234,8 @@ int xbps_transaction_update_pkg(struct xbps_handle *xhp, const char *pkgname, bo
  * @retval EBUSY The xbps package must be updated.
  * @retval EEXIST All installed packages are already up-to-date.
  * @retval EINVAL Any other error ocurred in the process.
+ * @retval ENOENT No packages currently register.
+ * @retval ENOTSUP No repositories currently installed.
  */
 int xbps_transaction_update_packages(struct xbps_handle *xhp);