To display job titles on payslips, assign the employee to a department. The section to add a department under the employee profile > Edit Details > Employment section > Department(s):

You can add a note directly onto the payslip to display additional information. To add a note on the payslip click on 'Add' in the Notes section of the open payslip:

The note is then shown as below on the payslip

To show the job description and / or notes on employees' payslips, go to Create/Send Payslips > Download PDF Payslips for multiple employees > select the 'customise' tab - ensure that the correct fields are enabled and save.