Bug 17697: Improve NotesBlacklist system preference description

Bug 17697: Improve NotesBlacklist system preference description

- Switched 'separator' for more common 'tab' and rephrased sentence
- Added examples
- Added note to use hidden settings for hiding in other spots.

To test:
- Search for NotesBlacklist system preference and read the description
- Apply patch
- Refresh and read again
- If improved, sign off.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>