Bug 18800: Patron card images: Add some more explanation to upload page and fix small...

Bug 18800: Patron card images: Add some more explanation to upload page and fix small translatabiity issue

The file
koha-tmpl/intranet-tmpl/prog/en/modules/help/patroncards/image-manage.tt
has a small translatability issue (sentence splitting by html tags).

This patch fixes it and adds a little bit more explanation about
uploading, using and replacing such images.

To test:
- Verify that text changes make sense
- Apply patch
- Go to Home > Tools > Patron card creator > Images and verify
  that the page displays properly

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>