Bug 15926: Add help for item search fields admin

Bug 15926: Add help for item search fields admin

This patch will add the missing help file to the item
search fields admin page.

To test:
* Go to administration > Item search fields
* Click Help
* Confirm help is there and correct.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com
(cherry picked from commit ac3e13248abd9687bbce764d1f1b6be39921b552)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>