The city shall have the power to borrow money for the purpose of constructing, purchasing, improving, extending or repairing of public utilities, recreational facilities or any other self-liquidating municipal function not prohibited by the Constitution and laws of the State of Texas, and to issue revenue bonds to evidence the obligation created thereby. Such bonds shall be a charge upon and payable solely from the properties, or interest therein, pledged, or the income therefrom, or both, and shall never be a debt of the city. All such bonds shall be issued in conformity with the laws of the State of Texas.