Bug 14394: fix documentation of OpacHiddenItems

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>