It is declared that revenue bonds, issued under the provisions of the Water and Sanitation District Act, except for those general obligation bonds described in section 73-21-43 N.M. Stat. Ann., shall not be considered to be general obligations of the district issuing them and shall be collectible only out of the net revenues derived from the operation of the water or sewer system, or joint water and sewer systems or from services rendered by the district for a fee, whose income is so pledged. Each of the bonds of any issue of revenue bonds issued under the provisions of the Water and Sanitation District Act, with the exception of general obligation bonds, shall recite on its face that it is payable and collectible solely from the revenues derived from the operations of the water or sewer system or joint water and sewer system or from the services rendered by the district for a fee, the income of which is so pledged, and that the holders of the bonds shall not look to any general or other fund for the payment of principal and interest of the obligation.