Bug 10878: Correct Display856uAsImage pref description

Bug 10878: Correct Display856uAsImage pref description

Removes note about Display856uAsImage not working on
the OPAC result page.

To test:
- catalog a record with 856$u = URL to an image, $q = img
- turn on the system preference Display856uAsImage
- check that the pref description makes sense and is correct
  - the warning about the pref not working on result pages
    has been removed
- make sure your record has been reindexed by Zebra
- verifiy the image indeed displays on the result page
  in the bootstrap catalog.

Note: The height=100 doesn't work in the Boostrap catalog,
so the images display in their original size. Will file
a separate bug for this.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>