Bug 14424: Add on site checkouts help for 3.20

Bug 14424: Add on site checkouts help for 3.20

There is no help for the pending on site checkouts report. This patch
adds it.

To test:

* visit pending on site checkouts
* click help
* confirm help is there and correct

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
(cherry picked from commit c00c6e3c675da7f244d5042e1ba9be4648bd4ee9)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>