Bug 11257: (followup) Document <<items.content>> for DUEDGST and PREDUEDGST and updat...

Bug 11257: (followup) Document <<items.content>> for DUEDGST and PREDUEDGST and update sample notices

This patch changes the default text for the DUEDGST sample notice to the
following:
    Dear <<borrowers.firstname>> <<borrowers.surname>>,
    The following item(s) are due:
    <<items.content>>
(it does not include an atomic update)

Test plan:
1. Select "edit" on DUEDGST in the notices and slips page (Home ->
        tools -> Notices and slips)
2. Open "email", and note the default text.
3. Apply the patch, and peform a fresh install of Koha.
4. Navigate to the same page. The default text should have changed.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>