Though there is a 1-to-1 relationship between database documents and data sources,
each of the two can exist without its counterpart, but create this counterpart on request
only. As a consequence, the document obtained via this attribute might be newly created,
which implies that the caller is now responsible for it. In particular, the caller is
responsible for calling ::com::sun::star::util::XCloseable::close
on the document as soon as it's not needed anymore.
Additionally, if the caller does long-lasting processing on the document, it's advised
to add itself as ::com::sun::star::util::XCloseListener to the document,
to prevent closing as long as the processing lasts.
