Fix doc errors in ml files and reactivate ODOC_WARN_ERROR

The following discussion from !4973 (merged) should be addressed:

• @raphael-proust started a discussion:

  This needs to be reverted once all the new errors have been fixed.

  The new errors have appeared because the new version of odoc now also checks the docstrings in the .ml files. This wasn't the case before and so some errors were just undetected.