Bug 27936: Clarify AllowItemsOnHoldCheckoutSIP syspref help text

Bug 27936: Clarify AllowItemsOnHoldCheckoutSIP syspref help text

The waiting reserves have always been impossible to checkout via
SIP2. There was some confusion whether this is actually true but if
you check the commit "Bug 25690: Remove duplicated logic in SIP2
checkouts" (338ba1d769) and the removed code around line

  $self->screen_msg("Item is on hold for another patron.");

you will be sure that it has been impossible to checkout waiting holds.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>