When printing payslips, from the selection screen you will see the following option called ‘Message for each payslip’
Any text typed in here, will be printed on the bottom of all payslips, or included on the payslip emails.
If you need a note to go on only one payslip, that worker would need to be in a transaction on their own, so you could print their payslip separately from the rest and therefore enter in the message above just for them.