(a) The board of trustees may:
(1) incur indebtedness;
(2) on the credit of the retirement system, borrow money to pay expenses incident to the system's operation;
(3) renew, extend, or refund its indebtedness; or
(4) issue and sell negotiable promissory notes or negotiable bonds of the system.
(b) A note or bond issued under this section must expressly state that the note or bond is not an obligation of this state.