Format() runtime function help (https://help.libreoffice.org/Basic/Format_Function_Runtime) should contain a link to general format codes (https://help.libreoffice.org/Common/Number_Format_Codes).
Yeah... pretty intense textual description of formats.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/help/commit/?id=769876a87a8ebbdfa80a0bfa9bab11fcbf56a83c tdf#114418 BASIC Format() function help page
Olivier Hallot committed a patch related to this issue. It has been pushed to "libreoffice-6-0": http://cgit.freedesktop.org/libreoffice/help/commit/?id=a5712dab8209d675b5e493b4dd64fd1300997755&h=libreoffice-6-0 tdf#114418 BASIC Format() function help page
Thanks Olivier! A nitpick: the format codes page isn't specific to Calc; it's the general format description that equally relates to Calc, and Basic (and maybe other places that use the same engine). It is placed under Common (or GLOBAL in new help system when using search in Index); its URL is text/shared/01/05020301.xhp (I suppose that "shared" also means not specific to single module).