Bug 31155: Document --since option in help of borrowers-force-messaging-defaults.pl

Bug 31155: Document --since option in help of borrowers-force-messaging-defaults.pl

This patch adds a section about the --since option in the help section
of borrowers-force-messaging-defaults.pl

To test:
0) Apply patch
1) run the script with the --help option, e.g.
./misc/maintenance/borrowers-force-messaging-defaults.pl --help
--> There should be an explanation of the --since option with examples
    for specific and relative dates
2) Optionally, run the script with different options
--> The behaviour shouldn't have changed

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