(9) The General Assembly may authorize the State or any of its agencies, authorities or institutions to incur indebtedness for any public purpose payable solely from a revenue-producing project or from a special source, which source does not involve revenues from any tax but may include fees paid for the use of any toll bridge, toll road or tunnel. Such indebtedness may be incurred upon such terms and conditions as the General Assembly may prescribe by law. All indebtedness incurred pursuant to the provisions of this subsection shall contain a statement on the face thereof specifying the sources from which payment is to be made.