Bug 11228: Improve hint on patron notification when adding/editing subscriptions

Bug 11228: Improve hint on patron notification when adding/editing subscriptions

The hint was not quite correct, as it said patrons on the routing list
would be informed, but it's only the patrons that have subscribed to
email notifications in the OPAC who will receive the email when a
new serial issue is received.

To test:
- Add a new subscription, find the hint about patron notification on the form
- Apply patch
- Refresh page
- Verify the new hint text makes sense and is correct

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>