Fix fdo#80442, Calc DBCS functions

write help pages for LEFTB, LENB, MIDB and RIGHTB spreadsheet functions working on DBCS

Change-Id: I31e42712f8e8ccbc8ad5ef4f933fcb7974489aa5
Reviewed-on: https://gerrit.libreoffice.org/9970
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>