Bug 20569: Improved description of CheckPrevCheckout system preference

Bug 20569: Improved description of CheckPrevCheckout system preference

The CheckPrevCheckout preference can be overriden by a setting in the
patron categories, I've edited the options to reflect this.

Test plan:
1. Check the CheckPrevCheckout system preferences and confirm that the
options are clear

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>