Context:
There is an issue described in OPT-3269, which leads to problems using cron expressions in Optimize. To make the user aware of these issues, we should write that down in the documentation.

AT:

• the documentation of cron trigger fields (`import.identitySync.cronTrigger` & `historyCleanup.cronTrigger`) is adjusted:
  • the default values are adjusted to be hour based instead of minute (as they are now due to the bug)
  • from 2.3 - 2.7 the default values being wrong in the distribution is documented in known issues of the upgrade documentation
  • the spring cron expression format is documented as being supported