Man pages matching name xmmultilistunselectallitems in section 3:

XmMultiListUnselectAllItems.3.en	debian 10 / libmotif-dev
XmMultiListUnselectAllItems.3.en	debian 11 / libmotif-dev
XmMultiListUnselectAllItems.3.en	debian 12 / libmotif-dev