The bonds authorized to be issued pursuant to the provisions of RCW 28B.20.700 through 28B.20.740 shall not be general obligations of the state of Washington, but shall be limited obligation bonds payable only from the special fund created for their payment as herein provided. The legislature may provide additional means for raising money for the payment of interest and principal of said bonds. RCW 28B.20.700 through 28B.20.740 shall not be deemed to provide an exclusive method for such payment. The power given to the legislature by this section to provide additional means for raising money is permissive, and shall not in any way be construed as a pledge of the general credit of the state of Washington.