UNO Date/Time conversion functions in "Converting Date/Time Values" help pages

Change-Id: I67608ec843a45b5a342c82a816174acb9391ca73
Reviewed-on: https://gerrit.libreoffice.org/6577
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>