Bonds issued under the provisions of this article shall be payable solely from the revenues derived from such waterworks or electric power system, and such bonds shall not in any event constitute an indebtedness of such municipality or county within the meaning of any Constitutional or statutory provision or limitation, and it shall be plainly stated on the face of each bond that the same has been issued under the provisions of this article, and that it does not constitute an indebtedness of such municipality or county within Constitutional or statutory provision or limitation. Subject to the provisions of subsection (b), section twelve of this article, the ordinance or order authorizing the issuance of the bonds may contain such covenants and restrictions upon the issuance of additional revenue bonds thereafter as may be considered necessary or advisable for the assurance of payment of the bonds thereby authorized and as may thereafter be issued.