Fix fdo#80440: help entry for ERF & ERFC.PRECISE

Write help pages for  ERF.PRECISE and ERFC.PRECISE spreadsheet functions

Change-Id: I6392684d91d4b43d954b3ba9b23db9d931f3e984
Reviewed-on: https://gerrit.libreoffice.org/10034
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>