Bug 15110: Improve decreaseHighHolds system preference description

Bug 15110: Improve decreaseHighHolds system preference description

To test:
1) Apply patch
2) Go to Koha admin
3) Search in system prefernces for 'high demand'
4) Verify that the system prefernce 'decreaseLoanHighHolds' is in the
results of the search

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit fe2dceba34310c890dd1f06b114de9f3151f59e1)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>