The page https://docs.camunda.org/manual/latest/reference/bpmn20/events/timer-events/ contains a link to Quartz in which cron expressions should be explained. This link is broken. I did not find any place in the forum where I can address issues of the documentation, so I hope it is correct that I placed it here.
thank you for the hint! You can have a look at the Quartz docs here: http://www.quartz-scheduler.org/documentation/quartz-2.1.7/tutorials/tutorial-lesson-06.html.
I will create a pull request to adjust the documentation.
Hope this helps, Ingo