fdo#75609 make Calc drag-and-drop help more detailed

Change-Id: I13c66772623c4749e48ae2d4746894a06e388ecb
Reviewed-on: https://gerrit.libreoffice.org/8524
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>