Bug 11120: Follow-up: adding a hint about the date format

Bug 11120: Follow-up: adding a hint about the date format

Adding 2 hints about the expected date format to the error
message and the help.

To test:
- Run overdue_notices.pl --date <someinvaliddate>
- Run overdue_notices.pl -man
- Verify hint about date format shows up

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit e6d987180fc30b4f5e89fa72f410b83185855f7b)
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>