Bug 10015 - UniqueItemFields could have link to database documentation

Bug 10015 - UniqueItemFields could have link to database documentation

Other system preferences which require entry of database column names
include a link to the scheme for reference. The UniqueItemFields should
do the same. This patch rewords the preference description to
incorporate a link.

To test, open the acquisitions tab in system preferences and confirm
that the UniqueItemFields description looks correct and links to the
right page in the schema.

Signed-off-by: Frederic Durand <frederic.durand@unilim.fr>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>