fdo#63274 clarify hexadecimal argument in HEX2DEC and similar functions

Change-Id: Ic8a4beb3f544be25706bd3f8c00beac150354fa8
Reviewed-on: https://gerrit.libreoffice.org/9906
Tested-by: Andras Timar <andras.timar@collabora.com>
Reviewed-by: Andras Timar <andras.timar@collabora.com>