Where could I get complete documentation of zen-cart functions?

When I looking inside zen-cart code, I found many zen-cart functions such as: zen_get_module_directory, zen_image, zen_display_banner, etc.

Is there a documentation for the functions?
If so, where could I get it?

Have you looked at the Wiki link in the top right corner here? There is a Developer's section that might help (I haven't read through it all myself as I'm not a developer) http://www.zen-cart.com/wiki/index.php/Developers

Thanks.
It still lacks of description and no downloadable version. Hope there would be an improvement in the future.

If it's code-level documentation you seek, the following may be of interest: http://docs.zen-cart.com/