Fix fdo#80443: Help pages for COVARIANCE functions

Help page for COVARIANCE.P and COVARIANCE.S spreadsheet functions

Change-Id: I130ec30ece2ec31cc540ab9e651140b2ea1ed07c
Reviewed-on: https://gerrit.libreoffice.org/10567
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>