Bug 19255: Correct explanation about list types on shelves.pl in staff

Bug 19255: Correct explanation about list types on shelves.pl in staff

The explanation on list categories on the shelves form is removed.
The online help text is actualized.

Test plan:
Go to Lists in staff. Edit a list. The remarks about private and public
lists on the right side are gone.
Click Help. Read the adjusted text. Does it make sense?

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>