Bug 26880: Add explanatory text to add course reserve pages

Bug 26880: Add explanatory text to add course reserve pages

To test:
- go to course reserve add item and batch add items pages
- batch add has a little bit of explanation, add has none
- apply patch, restart, reload pages
- both pages have more text explaining how they work!

"Checking the box next to the field label will enable changes to that field. Leave boxes unchecked to make no change.<br>
Any items with existing course reserves will have their on reserve values updated."

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Kelly McElligott <kelly@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>