Most (?) of the built in functionality currently lacks corresponding documentation. The functionality to grab and attach the docs is there, the docs just haven't been added.

This, I believe, could make a good first issue for many people, it just involves taking the documentation and adding it before (after works too but most current examples add it before) the #[py(function/method/class)] attribute. As an example, for list.append one could just add:

/// Append object to the end of the list.
#[pymethod]
pub(crate) fn append(&self, x: PyObjectRef) { ... }

The main built-in classes seem to have documentation for the classes but not for the methods. Builtin modules, I'd assume, probably also have many more cases.