I've not been deeply involved in either so take my hunch for what it is:

OCaml is an academic language like Haskell. Academics tend to neglect documentation in general, be it because what they're working on is mostly self-serving, lack of time/enthusiasm or a tinge of elitism on that "this should be obvious and if not just read the code".

Rust adherents seem to really want to take over the world and they realize that great documentation is an enabler for that.

Also the standard libraries and compiler set a pretty strong precedent.