Bug 14394: fix documentation of OpacHiddenItems

The current documentation of OpacHiddenItems told people to go and read
a file on the server, which most people don't have access to. This
replaces it with a link to the wiki.

http://bugs.koha-community.org/show_bug.cgi?id=14394

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>

It doesn't apply for some reason. Fixed
Added target attribute to open in new window/tab,
hope you don't mind.

Updated documentation
No errors

Belongs to Aleisha or Robin?
Update assignee please :)

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
(cherry picked from commit 865321f372)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>

koha-tmpl/intranet-tmpl/prog/en/modules/admin/preferences/opac.pref

@@ -532,7 +532,7 @@ OPAC:
             - pref: OpacHiddenItems
               type: textarea
               class: code
-            - Allows to define custom rules for hiding specific items at opac. See docs/opac/OpacHiddenItems.txt for more informations
+            - Define custom rules to hide specific items from search and view on the OPAC. How to write these rules is documented on the <a href="http://wiki.koha-community.org/wiki/OpacHiddenItems" target="_blank">Koha wiki</a>.
         -
             - pref: OpacAllowPublicListCreation
               default: 1