Bug 35354: Update description of EmailLibrarianWhenHoldisPlaced system preference

Bug 35354: Update description of EmailLibrarianWhenHoldisPlaced system preference

This includes more information about the email addresses
used in the system preference description.

To test:
* Search for EmailLibrarianWhenHoldisPlaced in administration >
  system preferences
* Verify description
* Apply patch
* Verify improved description and that the links work as expected

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>