VBA Function FormatNumber was recently implemented, this bug is to write the help page for it. See bug#118218.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/+/f33281a5c92c9daa241979cd461c21f3e5cec309%5E%21 tdf#118473 Help page for VBA FormatNumber