... but its documentation claims that it is, which is partially
misleading, so fix both the documentation and the data race in
WeakReferenceHelper::clear().
This actually crashed in clear() in the multi-threaded ZipPackage code
on exporting the bugdoc from tdf#94212, presumably because clear()
races against OWeakRefListener::dispose().
Change-Id: I85665c11b8157e90d15e8263758e24e66efeb86c