refentry.pagebreak — Start each refentry on a new page
If non-zero (the default), each refentry
element will start on a new page. If zero, a page
break will not be generated between refentry elements.
The exception is when the refentry elements are children of
a part element, in which case the page breaks are always
retained. That is because a part element does not generate
a page-sequence for its children, so each refentry must
start its own page-sequence.