I agree that the usefulness of documentation systems such as odoc comes from a carefully curated interface, but I disagree with them needing a separate file which in my experience is cause of friction.

Other languages (I am familiar with Rust and Python) also have documentation systems that only show you public methods (and functions, types, etc.), and I find them equally useful as odoc. They just take the visibility information from inline annotations (which are enforced by the compiler, in the case of Rust) instead of a separate file.