Bug 35996: Make it clear that --category-code can not be used alone

Bug 35996: Make it clear that --category-code can not be used alone

This patch adds a clarification to writeoff_debts.pl to make it clear that --category-code can't be used as the only filter when running the script. If this is the case, the script just displays the help menu as it is expecting one of the other filters

Test plan:
1) Run perl misc/cronjobs/writeoff_debts.pl --category-code TEST --confirm
2) Observe that the help menu is displayed instead of running the script
3) Check the help menu for the text added in this patch

WNC amended patch - added same warning in the option section

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>