Fix fdo#80449: help text for CONFIDENCE functions

CONFIDENCE.T and CONFIDENCE.NORM

Change-Id: Ie63eddc86dfd1de116acc980d5d672678279e779
Reviewed-on: https://gerrit.libreoffice.org/10408
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>