Does the .mli file preserve documentation comments? IMHO the header file should be written for human consumption first. Documenting the implementation isn't by far as important.
The main OCaml implementation doesn't, but I believe there are several third-party tools that automate this. And as you've mentioned, comments in `.ml` and `.mli` do tend to differ, so even the default auto-generation may be useful because it will do as many jobs as possible without a human intervention.