Bug 12592 - Minor correction for C4::DelBiblio documentation

The function header says that DelBiblio checks to make sure there aren't
any issues on any of the items. What the code does, on the other hand,
is to check whether biblio has any items attached, and refuses to
delete biblio if it has any.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>