Bug 31854: Document --not_borrowed_since and anonymization

Bug 31854: Document --not_borrowed_since and anonymization

To test:
- Run "perldoc misc/cronjobs/delete_patrons.pl"
- Note the absence of information about the potential conflict
  between --not_borrowed_since and anonymization
- Apply this patch
- Re-run the perldoc command and make sure the note about
  anonymization make sense

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>