The function ISOWEEKNUM is missing in online help https://help.libreoffice.org. Should be in this section: https://help.libreoffice.org/Calc/Date_and_Time_Functions ISOWEEKNUM help exist in local help.
Checked and confirmed.
Fixed by the latest upload job: https://help.libreoffice.org/5.1/Calc/ISOWEEKNUM https://help.libreoffice.org/5.2/Calc/ISOWEEKNUM https://help.libreoffice.org/Calc/ISOWEEKNUM