The tricky bit is most languages I’m familiar with have specific modules from their standard library that get automatically pulled in, and so they just document what those modules are. For Python its builtins — Built-in objects — Python 3.12.3 documentation and for Rust its std - Rust and specifically the std::* section (new users are capped at 2 URLs, so I didn’t keep looking for more examples). ReScript doesn’t seem to have an equivalent “we call open automatically on this module from Core”, instead having specific things like option and result.
If result is the only thing missing, then maybe just expand “Null, Undefined and Option” to include it? Otherwise it’s like there’s a need for “Other Types” or “Non-Primitive Types” page.